NOTE: This spoiler was sent in by Brentage5000.
The movie starts in lower Earth orbit on October 3, 2004, around a satellite belonging to Weyland industries. We see some satellite data and then we head for Weyland industries in Nevada, where some researchers realize that the satellite has just picked up a bloom of heat...in Antarctica. Cut to Nepal, where we see Alexa "Lex" Woods (Saana Lathan) climbing up a glacier. She is interrupted by a phone call from a man named Maxwell Stafford (Colin Salmon) who works for the Weyland corporation. He tells her that her services are required and a substantial donation will be made to her company but she says no...until she gets to the top and finds Stafford already there with a helicopter waiting for her.
Meanwhile in Teohiticualan, Mexico, we meet Sebastian (Raoul Bova) and Thomas (Sam Troughton), two men trying to find an ancient burial site. Sebastian digs in the ground a little, goes inside a hole while Thomas films, and discovers...a long-buried 1970s Pepsi-Cola bottle cap. Disgusted, the two go back to the tent where who should they find but Stafford, who makes them the same offer he made Lex -- come with me to meet my boss and find something you'll go gaga over.
A day or two later, we see Lex aboard a helicopter with a man named Graeme Miller (Ewan Bremner). A family man, he asks Lex to take a photo of dad looking cool for his two sons but he ends up looking to the side for it. They talk but are interrupted by the pilot announcing that they just passed the POS. Miller tries to get a picture of it until Lex explains that POS means point of safe return. When Miller asks what happens if they need to ditch, Lex gives him a big speech that basically says, "We freeze to death."
Later we see Miller, Lex, Thomas, and Sebastian aboard a ship with a bunch of other people. There we finally meet Charles Bishop Weyland (Lance Henriksen). He tells them about the heat bloom in Antarctica and that his scientists have determined the bloom is at a pyramid buried about 2000 feet beneath the surface. He offers up a map based on what it supposedly looks like and Sebastian says it's the same style as the Aztec and Mayan cultures. Weyland intends to be the first person down there since other people will likely notice it before long. Everyone is eager to get down there except Lex, who says that it will take at least three weeks to get everyone trained properly. Once Weyland says that they don't have that kind of time, Lex quits.
In space, in a dark circular room, an image flickers on of a pyramid. A circular shape appears on the pyramid followed by a triangular one higher up beyond where the pyramid is. A line goes down from the triangular shape to the circular one as we pull back and see three unworn Yautja (re: Predator -- I'll be using both names) masks.
Back on Earth, Lex is standing on the ship watching some weird lights in the sky. Miller and Sebastian come over to join her and try to persuade her to stay but fail. They are interrupted by Weyland's head of security Adele Russo (Agathe De La Boulaye) who tells Lex the helicopter is almost fueled up. Later Lex is finishing up packing but she is interrupted by Stafford, who tells her that money has been donated and she can leave any time. For some reason Lex then asks Stafford who they got to replace her and he says, "Daniel Woods." Apparently, though, Lex doesn't like this because the next scene is her telling Weyland that Woods isn't qualified enough but Weyland says that the other guys he called both gave him the same bullshit answer Lex did. Lex pleads with him to reconsider but he doesn't so instead Lex ends up staying. However, before anyone goes off on the ice she institutes three rules -- no hero stuff, no funny business, and no going off alone. She says this especially to the head digger, thinking that he might be trouble, I guess. As everyone saddles up, Lex sees Adele packing a gun. She tells Adele that she wont need it, but Adele compares having the gun to having a condom -- "Id rather have one and not need it than need it and not have one."
Once out on the ice the team finds an old whaling station that's been abandoned since 1904. The digging team sets up and starts working their way down while everyone looks around. Miller, Lex, and Sebastian head off in one direction but as they're exploring, the picture-happy Miller heads off for some shots. He finds himself in a little kitchenette that is nicely separated from everywhere else. He takes a couple shots with his camera when he hears a very small thing moving about rather quickly behind the pots and ovens (a facehugger?). He bends over to look and is scared straight (get it?) by Lex, who slowly says no one goes anywhere alone. Miller points out the thing behind the whatevers and suddenly...a little penguin comes out from behind the stove. While Miller and Lex are laughing, Sebastian yells out about something he's discovered -- a huge tunnel leading straight down through the ice to the pyramid The head digger says that nothing on Earth could have done that to the ice but that doesn't stop him and his men from setting up a rig to go down. Before they leave Lex goes to find Weyland and discovers him sitting in a rig breathing on an inhaler. Apparently, he has a lung condition but he assures Lex that he'll be alright.
Meanwhile the map of the pyramid on the Yautja ship from before is now showing various scattered human images on it. The three masks stare sightlessly at this until big clawed hands pick them up and put them on. We then see the hands grabbing wrist attachments and ejecting the blades from them, trying them on for size. As they are doing this the big ship begins to enter the atmosphere. We cut to see Weyland, Lex, Adele, Thomas, Sebastian, Stafford, Miller, and some troops working their way down the tunnel. Meanwhile the ship passes over the heads of the digging crew with nary a one of them noticing as it flies by...or as it drops off three pods which hit the surface just outside of town...and then open...
Back in the tunnel, things are moving slowly when suddenly Weyland slips and starts sliding past everyone -- past troopers Verheiden and Connors, past Sebastian and Thomas, and almost past Lex, until she brings her pickaxe down at the last second through his hood, barely saving his life (and barely scalping him too, I imagine). They get down the rest of the way and are greeted by the sight of...well, of darkness. But once they get some power generators up and running they can see that they're outside a pyramid just like the Aztec ones. They go inside past some statues of some Yautja hunters (okay, they don't realize that's what they are, but...) and as they're examining the walls somebody steps on a switch that activates some machinery in a room far below...machinery that brings up a long dormant Xenomorph (Alien, but I'll just call them xenomorphs) hive queen and makes her start laying eggs onto some conveyor belt. Meanwhile, the humans have started looking around and have come to a room which Thomas identifies as the sacrificial chamber. When Adele notes the condition of the chest cavities of the corpses, Thomas points out that heart pulling (or whatever he said) was a big part of sacrifices. Adele just looks at him and says, "That's not where your heart is." She also points out that the bones are bent outward meaning something came out of him. They then realize that there's another room below them, so Weyland says that Thomas, Adele, and some others will stay but the rest of them -- Connors, Sebastian, Lex, Miller, Verheiden, and Stafford -- will come to the second room.
Up on the surface the diggers are trying to stay warm and kill time when they are attacked...by nothing? They watch as one of them struggles on a spear when a door bangs open -- supposedly by the wind -- but are then killed quickly and viciously by the three Predators. The head digger runs and dodges until he finds a gun but before he can attack he stumbles and goes sliding down the tunnel, unable to find anything to slow his 2000 foot slide. At the bottom he hits a wall of snow and lies there hurting and miserable...and watches as the three Predators come down. Two of them head straight for the pyramid and he breathes a sigh of relief...until the third one's knives extend and kill him.
Back in the temple, Weyland, Stafford, Connors, Lex, Miller, Sebastian, and Verheiden have found the lower chamber. Inside it are more hieroglyphs and a large coffin. Sebastian looks at it and notes that the combination lock has a date on it for 1904...the year the station got abandoned. He puts in the day's date -- 10/10/2004 -- and the sarcophagus opens revealing three strange objects. Lex says that they should mark this room and head back for camp but Stafford ignores her and he and Weyland take the things out despite Sebastians protests...which they should have listened to because the second they're out the walls start switching around, which locks the people inside the sacrificial chamber in there and makes a set of stairs descend from the lower chamber. In the sacrificial chamber, Adele tries to hold the door open with a box that can absorb two tons but buckles as if it were paper. Trapped in the room, the people have no idea what to do...which is just when some eggs come up above the tables in there. Adele asks Thomas what this room was and his terrified response is two words: "Sacrificial chamber." Right on cue, the eggs open and all of them are attacked and infected by the facehuggers. Later, we see Adele wake up next to a dead facehugger. She screams at it but quickly reassures herself that it's dead...which is when she notices the facehuggers on everyone else in the room. Before she can react, she begins to writhe in agony about five seconds before a chestburster pops out of her chest, killing her instantly.
Back with the noninfected humans, they are making their way through a great hall that Lex says should take them right to the exit. Suddenly they are attacked sniper style by one of the Predators, but before he can do anything more the walls shift again, splitting them into three groups: Miller and Verheiden; Connors and a bunch of xenomorphs that kill him instantly; and Weyland, Sebastian, Stafford, and Lex. We hang out with Miller and Verheiden for a bit and discover that they both have sons, which to Miller is reason enough to try damn hard to make it out of there. With Weyland's group we see Sebastian theorizing that since the Mayan calendar was based on ten the walls should rearrange themselves every ten minutes. Lex asks how long they have to wait...which is right when the walls start moving, making Sebastian shrug before they all head forward through the now open door. Unfortunately, the latest move has less than stellar consequences for Miller and Verheiden, who are separated by the walls and attacked by xenomorphs.
Back with Lex and company, they are trying to make it to the exit when something shoots a net at Stafford, pinning him to a wall. They try to get him free but are interrupted by the yautja that fired the damn net in the first place. He bats Lex across the room and right into the path of another Predator, who extends his knives...and then gets impaled by a xenomorph tail. The other yautja sees this and forgets about the humans, which lets them get out while the xenomorph knocks the second yautja down. The Predator throws the xeno off and then rolls out from under it managing to slice off the end of its tail. The xeno runs off and the yautja gives chase but loses him. The Predator looks around and the xeno jumps out but is caught by a net. The xeno breaks free of that though, and knocks the Predator onto a pile of bones in the room below. There it pins the yautja down and kills it by spearing it through the eyeholes of the Predator's mask with its extendable tongue.
During all this, the remaining humans are desperately trying to escape when suddenly the last remaining Predator appears behind them. Weyland, gasping and likely dying, tells Lex and Sebastian to go on ahead. They do and Weyland gets ready to make his final stand. The Predator almost kills him, but it uses it's heat vision to zoom in on Weyland's lungs and realizes the old guy is sick. The Predator puts Weyland down and heads for Lex and Sebastian but is...distracted...when Weyland turns a flare and a snowgun into a flamethrower, which gets him the attention he wanted and kills him in the process.
Meanwhile, Lex and Sebastian are running like crazy but are interrupted by another wall shift. The floor starts rising and they crawl like mad for the end of it with Sebastian narrowly being crushed and a shuriken of the Predators nearly killing him on top of that. Once he's down we see the Predator on the other side contemplating the situation until a facehugger jumps at him...although he promptly shurikens the little bastard, so it's okay Right after he catches the shuriken, he turns and slices the face off of a xenomorph that was creeping down behind him (really cool shot here of the front half of the xeno's head falling off). He bends over his kills and snaps off a finger from the fully grown xeno, then uses it to burn a mark on his mask and forehead (this is called blooding in the comics). Just as he finishes and turns a facehugger leaps off the wall behind him.
Back with our humans, Sebastian is reading hieroglyphs and realizing the purpose of the Predators presence. Every hundred years, the yautja would come to this backwater planet for the purpose of conducting a coming of age hunt for their teenaged members. The local humans who worshipped them as gods gave themselves as sacrifices so that the hunt could begin and if things ever got out of control the Predators would set off devices that would erase everything -- man, yautja, and xenomorph alike -- from that area of the world. Lex realizes that they need to give the weapon they found back to the Predator and work with him to kill the xenos -- "The enemy of my enemy is my friend," says Sebastian. So agreed, they get up just in time for the walls to shift and a xeno to come out of the woodwork. It chases them down a hall and across a broken stone bridge. Sebastian makes the jump okay but Lex falls short a couple feet. Sebastian tries to help her but is impaled from behind by a xeno. Lex ducks down and then looks back up to see...an empty hall. She climbs up and makes for a room at the end of the hall -- it's completely empty. Lex turns and sees the last Predator standing there. It lifts its spear but she holds her hands out and in a supplicating gesture lays the weapon at the feet of the Predator. It puts the spear down, picks up the weapon, puts it on...and we see that its the laser beam from the other two movies. He aims it at Lex and then turns and starts firing at xenos that are coming down the passage. Lex watches but does nothing...until a xeno starts attacking her. She ducks away from its tongue, picks up the spear, and rams it right through the chest of the xeno. Once all the xenos are dead or fled, the Predator moves to leave, but is stopped by Lex, who wants to come with him. After roaring at her and seeing she still wants to come, the Predator snaps off the tail and head of the xeno Lex killed. He sticks the tail on the ruined end of the spear, makes a harness for the head (Lex's trophy), and then sets off, now accompanied by Lex.
During this touching scene, the Queen has been calling to all her troops. She has them start killing each other on top of the chains holding her in place. Eventually, the acid blood melts through the chains, leaving one very free, very pissed off Queen xenomorph.
Back with Lex and her new friend, they are hunting for the main xenomorph nest. Lex notices Miller's camera on the floor and starts down a passageway. She finds a chestbursted Miller bound to the wall and a gun at his feet (he got it earlier just before being facehugged). Lex picks the gun up and sees Sebastian -- still alive and equally bound. She runs over and starts trying to cut him loose, but he tells her that he's got one of those things inside him. The yautja wants to kill him but Lex refuses until Sebastian's screams force her to it. Lex and the Predator turn to leave but before they do, the Predator points at a tool on his wrist -- one he's had all along -- and makes the international this-place-go-boom sign. Lex nods and the Predator sets it and throws it into the middle of the nest, which is becoming quite active. Lex and the Predator run back for the main entrance chased by a shitload of xenos, activate the transport to the surface, and ride up superfast, just barely beating the xenokilling explosion. Before they can rest, they realize the explosion damaged the surface, meaning that they have to run like hell out of town which they barely do.
Once all the excitements over, the Predator takes a look at Lex and slowly begins taking his mask off. Once he does he points to the mark on his forehead he made earlier. Lex turns her left cheek towards him and he makes the same T-shaped mark there. The two look at each other and then a loud noise makes them look back toward where the station was...and they see that the Queen is back in town, and she is none too happy. She roars at the unlikely pair and chases them, batting the Predator aside like a soda can. Lex runs for a water tower or something like it on the edge of a cliff and hides underneath. The Queen keeps trying to get to her but is meeting with little success. Lex sees a chain lying nearby that is running from the Queens neck but there is no way to get to it...until the Predator comes back and starts throwing his shuriken at the Queen like there's no tomorrow. Lex gets the chain tied just as the Queen manages to impale the Predator on her rather large tail. Regardless Lex tricks the Queen into pushing the tower the rest of the way over so it falls into the frigid water below, dragging the Queen along with it.
While the Queen is sinking, Lex is looking at the dead body of her comrade in arms when suddenly a host of other Predators -- along with a Predator ship -- uncloak (they were there and invisible the whole time the fight was going on...real helpful bunch, aren't they?). The head Predator, who is wearing armor and some kind of big red cape, looks at the dead Predator's body. He summons four other yautja to take it into the ship and then looks at Lex and, more particularly, the mark on her cheek. After a moment he gives her the dead Predator's spear, turns, and heads back into the ship, which takes off as Lex watches from below.
On the ship, we see a few Predators milling around. At the very front, we see that a funeral brier has been laid out with the young yautja atop it. As a few nearby yautja walk out of frame we zoom in on the body. Suddenly, a chestburster comes out of the Predator, and at the sides of its mouth we see four tiny tusks...