The film begins with the voice of an astronaut calling in a mayday back to base, he says that he was suppose to touchdown somewhere near white Sands New Mexico, but something went horribly wrong and he has gone through a tear in time and space. The astronaut hears something approaching him, and tries to run for it, but does not get far due to the soggy swampy terrain, and his bulky astronaut suit. He trips and falls into a muddy puddle, and looks up to see something large come at him. He screams, and the screen fades to black.
Cut to an intro for the Today Show hosted by Matt Lauer (played by Matt Lauer), interviewing a scientist about Quantum Palaeontology, Dr. Rick Marshall (played by Will Ferrell). Marshall believes that his field will solve the fossil fuel problem, leading Matt Lauer to think that its renewable bio-fuels, but is surprised to find that Marshall’s solution is “time warps”. Marshall has spent 50 million dollars of taxpayer money and has come up with nothing, and is asking for more funding. Matt Lauer admonishes him for this, but he just brushes it off and replies that there is much untapped trans-dimensional energy available. Lauer quotes Stephen Hawking as calling Marshall’s theories nonsense, and Marshall, offended, stomps off the stage as Matt Lauer ends the interview by talking about Marshall’s book being available in the “I’m out of my freaking mind department,” this causes Marshall to snap, and charge at Matt Lauer.
Some time later, Marshall is in a darkened room, giving a presentation about tachyons, a particle that travels faster than light and can pass back and forth through time. He tells his audience that harnessing tachyons should allow them to travel through time and space, and introduces his new invention, a “tachyon amplifier”, allowing the user to travel through time, not forward or backwards, but sideways in time, to another dimension where past, present a future meet. The lights come on, and it turns out that Marshall is just giving a presentation to a bunch of elementary school children, who completely ignore his theories and make fun of him (almost brings him to tears).
As the kids leave, a young woman approaches Marshall; her name is Holly Cantrell (Anna Friel) a palaeontology student from Cambridge, and a big fan of Marshall’s work. Holly urges Marshall to continue his work, but he has lost faith in it, resigning himself to a life of teaching elementary school children at the La Brea Tar Pits. To his astonishment, Holly presents Marshall with a 265 million year old fossil with the imprint of a lighter in it, along with crystals that emit pure tachyon energy, she found in the desert. Holly leaves to let Marshall think about it. Once Holly is gone, Marshall pulls out his own lighter and fits it right into the imprint; it was his lighter that made the imprint, meaning that sometime in the future, he goes back in time. He’s a bit freaked out.
The next day, Holly arrives at Marshall’s lab, and finds him passed out on the floor, having spent the night binge-eating. She finds that he has completed the Tachyon Amplifier. Marshal said he decided to complete the device, but did not have the courage to turn it on, and so he went on a food binge to try to build up some courage, which did not work. Holly continues to admire the device, calling it a work of genius and presses a button, causing it to blast show tunes. Marshall tells her that it’s just left over files from the hard drive he incorporated into the device, but notes that he’s also a fan of classic show tunes.
Marshall is still dejected, but Holly drags him off on an expedition to the desert to test out Marshall’s new invention. They arrive at Devil’s Canyon Mystery Cave, where she found the fossil and the crystals. The Cave is own and run by Will Stanton (Danny McBride), a redneck-like character who tries to sell them fireworks and cheap souvenirs, thinking that they’re tourists. He’s trying to save up enough money to build a casino. He offers to give them a tour of the cave if they buy $30 worth of stuff.
There is a river within the cave, and Will, Marshall, and Holly all get into a raft and Will steers them deeper into the cave. Will attempts to make the rather mundane raft ride interesting by giving a spooky commentary about lizard men. Will has to yell out the story several times until one of his employees pushes out a rubber lizard man dummy in an attempt to make the ride scary Holly and Marshal just stare at him. Soon after, Marshal’s device detects a large amount of tachyon energy, and attempts to boost the signal and open a doorway to the other dimension. This causes the river, which is in fact just a sewage line from the local noodle factory, to suddenly turn violent, rushing them deeper into the cave. All three of them are tossed about on the raft and Marshall drops his invention into the water, and a moment later, they go over a waterfall into a swirling vortex of light.
Some time later, they wake up in a very unfamiliar desert. Holly finds an airplane crashed into a Viking ship among other things that don’t belong in the desert. Marshall, realizing that he has finally proven himself to be right, yells out for Matt Lauer to “suck it!” Marshal talks into Holly’s video camera and describes their situation, and how they cannot return without the tachyon amplifier. Just then, they hear a distant hooting, and follow it to investigate.
They peer over a sand dune and see several primitive human-like primates. Two of the primates seem to be trying to sacrifice a third on an altar. Will attempts to scare them off with a lighter, but ends up burning his hand, dropping his lighter, and allowing the primates to run off with it. Holly, Marshall and Will find the primate they saved cowering behind the altar. Holly has experience with primates, and attempts to communicate with it, and it calls himself Chaka (Jorma Taccone), while touching Holly inappropriately. Marshall offers to look at Chaka’s injured leg, frightened, Chaka screeches at Marshall and pushes him away, then runs off, pursued by Marshall and the others.
They don’t get very far though, and all four are sucked into an underground cave through a sinkhole. After recovering from the fall, Holly figures out Chaka’s language (based on primitive earth primates), and is able to tell him that they’re friends. Chaka thinks Marshall is his new master for saving him, and gropes at him inappropriately. Marshall and Holly try to figure out what to do next, while Will freaks out (and freaks out even more when Holly and Marshal appear more interested in their predicament, rather than fearing for their lives). As they argue, all four are pulled up into air by green plant-like tentacles.
Marshal looks down and sees that the floor is full of skeletons, including what looks like an astronaut’s corpse (probably the one from the beginning of the film). Marshal figures out that it’s a feeding pit, and gets all of them to pull at each other to swing over to the ledge to escape before the owner of the feeding pit comes back. Unfortunately, a T-rex comes by and tries to eat them. The T-rex bites through the vines and they fall to the floor, and run for it. Chaka abandons them and runs ahead. Marshal thinks that Chaka has found them a bridge for escape, but after he crosses, Chaka tries to tip the stone bridge over. The other three make it across the, and the T-rex can’t walk the narrow bridge due to his size. However, Marshall makes fun of the T-rex about how small its brain is (size of a walnut), and how dumb it is, making it angry, and causing it to jump clear across the chasm in an attempt to eat Marshall.
All four of them run for it, followed close behind by the T-rex. They make it into a cave that is too small for the T-rex to enter, dejected, it stomps off. They name it Grumpy. They take a moment to take a breather and figure out what to do next. Suddenly, they hear music coming from deeper in the cave, coming from an old victrola (those things that look a trumpet glued to a turn-table).
They follow the music to the bodies of two WWII soldiers, sitting at a table a table, apparently playing chess until they died. Holly, Will, Marshall and Chaka prepare to settle in for the night, while Chaka continues his attempts to molest Holly. Holly talks to him some more, and it turns out that Chaka was a prince, but betrayed by those who wanted his throne, and also because he pooped into the village well.
Marshall tries to make Chaka sleep outside (like a dog), but he just hisses at him. Will tries to place an old crate to block the entrance of the cave, but Rick disagrees, saying that they shouldn’t place anything in front of the cave to draw attention to themselves, but Will notes that they already have a super intelligent T-rex on their asses. Marshall continues to talk about how dumb the T-rex is, and that its brain is the size of a walnut. They hear a loud crash from the outside, and go out to a large leaf bundle, with a giant walnut in it, about size of a small child. They look across the chasm, and see Grumpy glaring at them as it sinks back into the shadows it clearly has a vendetta against Marshall.
The next day, Marshall videotapes himself again, and talks about what they have to do when they run out of food, thinking about cooking Chaka, and going into detail about how to cook him properly. Holly and Will return with handfuls of fruit. Marshal tries to ask Chaka what food items are safe for consumption, and Chaka cracks open a coconut-like fruit and gives it to him. Marshall takes the fruit, but notices too late that it’s infested with hundreds of small spiders, causing a minor freak-out. Marshall tries to communicate his displeasure to Chaka, when a bright flash of light appears at the mouth of the cave. They run out to check it out and see another bright flash from the forest below. Marshall looks at the light and gets several mental flashes of a yellow lizard-like humanoid on the ground, saying something in an alien language. Marshal replies “I will,” and runs off, much to the confusion of the others. Will thinks that Marshall has “jungle madness,” and intends to put him out of his misery with a rock. Holly and Chaka run after him. Marshall runs through the jungle and stops when he finds a crystalline obelisk. Marshall notes that it’s some kind of beacon that has tapped into his mind, and ducks just in time to avoid a rock thrown by Will. The others catch up to Marshal and learn that he hasn’t gone made after all.
All four of them move towards the ruins where the beacon sits, while more lizardmen watch silently from a distance.
The four of them approach the beacon, an almost invisible crystalline triangle. Will puts his hands on it, and feels it vibrate, making his voice vibrate as well, and then starts singing a song by Cher. Marshall, approaches the beacon as well, stares at it in contemplation, then joins Will in singing.
Chaka starts freaking out, trying to warm them about something, but they’re all clueless this all happens while they’re standing in front of a written warning: “Beware of Sleestaks,” but they are oblivious to it. Chaka turns Marshall towards the wall, and dozens of the lizard-like Sleestaks come out and approach them. Marshall tries to tell them that they’re slow and not dangerous, although they’re completely surrounded and the Sleestaks don’t look friendly, opening their large mouths baring two rows of sharp teeth. Holly, seemingly the only brave one of the group, pulls out her belt and wraps it around her fist, intending to take down at least one of them before they eat her. As she waves her fist around, the sun reflects off her belt buckle and Marshal gets the idea of shining the reflected light at the beacon. This causes the beacon to emit a white beam towards other statues in the ruins, forming a small black triangular doorway that all four of them dive into. They think that they’re cornered inside the small pyramid, but turn around to find that the inside is huge. They look over and see a gold-coloured Sleestak, the one that called out to Marshall earlier. This Sleestak introduces himself as Enik (John Boylan).
Enik is a prisoner in the “Land of the Lost”, and tells him that Marshal is the only one who can free him, stating that the fate of the universe depends on it. Enik reveals that one of his species, called Zon is trying to harness the power of the crystals to open up portals to other dimensions and worlds for the purpose of conquest, including earth. Enik shows them a hologram of the Zon, warning of impending doom.
Enik tells them they Zon needs a device that that focuses and amplifies the power of tachyons, and Marshal tells him that he invented one, but lost it. Enik tries to track the device, but tells them that the device was moved, and that they must hurry to find it. Will says that they should never trust any dude in tunic (Enik wears a dress-like outfit). Enik tells them that if they want to go home, the crystals he possesses are the doorway, but Marshall’s device is the key. He tells them that he must stay in the pylon (the black pyramid) to guard the crystals, and sends them off to find the amplifier before Zon does.
Outside, Marshal builds a scale model of the place, and shows them his plan to find the tachyon emitter, which consists of tying Will to a hot air balloon to scan for the device. In order to avoid the T-rex, Marshal proposes covering themselves in dinosaur urine to make them undetectable. Marshal goes first, and even ingests some to get it into the blood stream. The other two are not at all enthusiastic about this plan. The urine starts to burn at Marshal’s eyes, and he douses himself again, making it worse.
Chaka appears a bit off into the distance and calls for them to follow him. Chaka leads them through the forest, and into an open plane. He leads them to a place that contains many of the world’s monuments (Mount Rushmore, the Golden Gate Bridge, and many others). Holly notes that this is probably the entry point to this dimension. They search for the device and find Will’s raft, but a moment later, an ice cream van appears and drops onto it. Will wants to go get his raft back, but Marshall stops him, telling him that all the little dinosaurs in the area are there for a reason, because they know its feeding time. Just them, several larger dinosaurs (raptors, I think), attack the ice cream van, trying to get to the man inside. They succeed and tear the man apart. A loud roar is heard, and the smaller dinosaurs are scared off by Grumpy the T-rex, and a female Allosaur. Marshall says that they’re very territorial and won’t notice them, but the T-rex smalls the urine on Marshall and chases after him. Marshal tries to run in a “serpentine fashion” to evade the T-rex, failing dismally. They split up to escape, and Marshall is cornered by both dinosaurs. Marshall hides among the mannequins of a store, but the T-rex finds him anyway (it’s smart). Fortunately for Marshall, the T-rex misses him and he takes the chance to run for it.
Marshal runs towards the other two now chases by the Allosaur, while carrying a canister of liquid nitrogen, yelling at them to arm a medieval catapult that’s behind them. Marshal loads the canister into the catapult, and they launch it at the Allosaur, which swallows it, causing it to freeze, inflate, and explode, sending chunks of dinosaur everywhere. A moment later, among the falling debris, is Marshall’s tachyon amplifier (apparently eaten by the dinosaur earlier). The device falls to the ground, still blaring show tunes. However, before they can reach it, a small dinosaur goes to pick the meat off the device, and is in turn, attacked by a pteranodon (or pterodactyl) and both the dinosaur and machine is carried away.
Exasperated, Marshall gives up and climbs into an old Cadillac, reclines, and sticks his hands down his pants (it’s his “giving up” stance). Holly tells him that even though everyone thought he was a joke, she followed his theories, and even got thrown out of Cambridge for it because she believed in him and his work, but now she realizes that it wasn’t worth it, and leaves.
At night, Will bonds with Chaka, talking about their lack of luck with women. Chaka talks about his old life, having Holly translate for him, but he appears to understand English. Chaka describes his life and society, and how he has 7000 wives who bring him stuff and serve his sexual needs, except that they’re hideous. Outside their little hut, Marshal is playing a banjo and returns to Holly, Chaka, and Will. He has decided to not give up (or, give up on giving up). He sings his apologies with song (badly), playing the banjo. Despite their polite protests and acceptance of his apology, he keeps singing, while a really large mosquito bites into his neck and sucks his blood. Marshal turns white from blood loss, as the mosquito inflates to the size of a water cooler with his blood. Marshal falls backward and crushes the mosquito, splattering blood everywhere, then passes out.
The next day, Marshal wakes up with a massive welt on his back where the mosquito bit him. The others wake up as well, with Chaka’s hands on Holly’s chest. Marshal gives them a stirring speech about the journey ahead, and set off to follow the music to the tachyon amplifier. They follow it up to the top of mountain, where the pterodactyl is incubating its eggs among a river of molten magma, with only a thin layer of volcanic glass separating them from the magma. Marshall tries to volunteer Will and Chaka to go get the device, but both feign unconsciousness. Marshal, having no choice, prepares himself, and then jazz dances his way to the device, avoiding the eggs. Just as he gets to it, the music stops, and all the eggs begin to hatch. Holly notes that the music was keeping the baby dinosaurs asleep, like a lullaby. Unable to get the device working, Marshal starts singing, joined by the others, and then Chaka breaks into the song in perfect English, before reverting back to his primitive gibbering.
They all return to the dimensional entrance, and take a swim in the pool of a motel that got transported to the Land of the Lost. Chaka grabs two weird-looking fruits from a tree and gives them to Marshal and Will, who both start drinking while Holly translates Chaka’s description of it. The fruit is a celebratory drink that appears to cause severe hallucinations and the sensation of being anally raped by zombies. Chakka, Will, and Marshall spend the next little while tripping out while Holly repairs the device and gets it working.
As Holly activates the device, another black triangular gateway forms and she leaves the others to explore it.
Back at the motel, Chaka, Marshall, and Will are still tripping out, and now have a bad case of the munchies. Off in the distance, a crab approaches them, as it gets closer, its turns out to be very big. It tries to attack them, but it crashes through the ground, and is spewed out of a geyser, steam cooked. A moment later, the three are squeezing a giant lemon on the cooked crab, while they talk about how much they like one another, and still tripping on their weird narcotic fruit beverage.
Will then ask Marshal how much he would have to pay to get him to French kiss Chaka. Marshal offers to do it for free, and they start making out.
Back with Holly, she’s exploring the cave, and finds what she thinks are dinosaur eggs among the crystals. She plucks one out to examine it, and turns around and faces Zon. It turns out to be a hologram, and complete one, not like the one Enik showed them. The hologram warns that Enik has escaped custody, stolen the central pylon and all its crystals, and intends on leading his army of rogue Sleestaks on a rampage across time and space. The hologram is about to warn them to not do something, but Zon was killed before he could finish the recording. Holly finds the body of Zon, but is captured by the Sleestaks.
The others finally recover from their drug trip and follow her trail to the triangular doorway. They sneak about and find two Sleestaks about to mate, taking off their skin to do so. Marshal and Will get an idea, and put on the skins to infiltrate the other sleestaks. At an altar surrounded by talking Sleestak skulls, Holly is put on trial for assisting Enik, and is about to be executed. Will and Marshall, having snuck in, pulls off their disguises to confront them, claiming to support Enik, which is a mistake, as it is revealed that Enik was responsible for the destruction of an entire civilization, and was imprisoned and forced to wear a tunic as a sign of his crimes (Will was right all along). These Sleestaks are the good guys, and the humans have been working for the wrong side. Marshal and Will are sentenced to death as well, but Marshal charges at Holly’s cage (suspended about a fiery pit that supposedly contains the Sleestak’s god) while Will distracts the Sleestaks. Marshal saves Holly, and she kisses him passionately, and tells him that she has feelings for him.
Holly tells them that they have to go stop Enik, unfortunately, Marshal, thinking that Enik was the good guy, had sent Chaka to go get him. Enik arrives with Chaka and his army of Sleestaks. Enik reveals that he uses the crystal around his neck to control his army of Sleestaks, and in weeks, the earth will be overrun. Enik tells him to save his strength, he’ll need it. Just them, Grumpy shows up for Marshall, and he faces off with it and instructs the other two follow Enik to stop him. Marshall tells Holly she doesn’t love her to get her to leave, including throwing a rock at her. Offended, Holly leaves. Will hands Marshall a large sack of fireworks he’s been carrying with him the whole time, to help him to fight the T-rex. Will tells him that he would follow Marshall into battle at any time, except for this one, and he leaves too.
Marshall grabs a stick and prepares to fight the T-rex while the others watch. Marshall grabs a firework and lights it, intending to shoot the T-rex, but all it does is give off a little pop. He drops his lighter and the T-rex stops on it, imprinting in into the ground.
Marshall remembers the motivational posture from his office (“If you don’t succeed, its your own damn vault”) and decides to vault over the T-rex, but only succeeds in landing right into its throat, and is swallowed.
Holly admonishes the T-rex for eating Marshall, and it stomps off.
Holly and Will run after Enik and must confront his army first. The Sleestaks are slow and rather easy to defeat, but there are just too many, and Holly is over run and captured. Seeing this, Will grabs Chaka, holds him like a battering ram and charges into the Sleestaks, and for a moment, appears to be winning, but is also overrun. A moment later, there is a loud roar, and Grumpy the T-rex arrives, ridden by Marshal. The Grumpy proceeds to tear through the army of Sleestaks with ease. Grumpy has taken a liking to Marshall and helps protect them from the Sleestaks while they ask Marshall how he got out of the T-rex, and after some badgering, Marshal reveals that he was pooped out of Grumpy. However, in the process of moving through Grumpy’s digestive system, he dislodged a blockage, improving Grumpy’s mood significantly. All four of them jump into the pylon after Enik, just as he prepares to open a portal to Earth. Marshal wrestles Enik and Will tries to help, except he knocks a crystal loose and breaks it, destabilizing the portal. Marshall grabs the crystal Holly wears around her neck and uses it to temporarily stabilize the doorway. Will volunteers to hold back Enik, along with Chaka to let Marshall and Holly escape.
Marshall and Holly walk through the portal and make it back to the cave on Earth.
Back in the land of the lost, Will brings Chaka back to his tribe and greeted like kings. It turns out that what Chaka described as hideous women, are actually very attractive (look like tanned bikini models).
Back on the Today Show Marshall is gloating to an embarrassed and humiliated Matt Lauer. Marshall gives Matt Lauer a gift, a dinosaur egg (which is really a Sleestack egg from the cave Holly went to) and is really enjoying himself proving him wrong with his new book, written with his partner, Holly, titled “Matt Lauer can Suck it.” Because Marshall is the guest on his show, Matt Lauer has to say the title of the book, which makes him angry, and he lunges at Marshall.
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