NOTE: This spoiler was submitted by L.
Peter Highmen (Robert Downey Jr.) is lying on his side in bed, leaving a voicemail for his pregnant wife explaining his latest dream. He dreamed that a bear cut the umbilical cord upon delivery and that instead of freaking out, he was OK with it. He tells her he loves her and goes about working through some architectural work for his company. Once he’s done he goes through baby names which he and Christine (Michelle Monaghan) and tells her that he doesn’t want to give his son a name that would be confused with a girl and leaves her another message. Christine calls him back and tells him that she can’t wait for his return.
Peter is driven to the airport in Atlanta. His cabbie stops in front of the airport and Peter gets out of the car with his luggage. Before he can close the car door, it is ripped off of its hinges by another car. Ethan Tremblay (Zach Galifianakis) gets out of the car and stands by Peter as the driver of the two cars fight over the damage. Peter notices that Ethan smells like booze and asks him if the other driver was drinking. Ethan tells him that he and the driver split a 6 pack of 30 beers and then knocks over Peter’s luggage. Peter asks him to pick it up but Ethan tells him that you can’t touch other people’s bags in an airport before walking off.
At the security check, Peter’s bag is flagged by security. They go through the bag and Peter realizes that he and Ethan swapped bags. A security guard finds marijuana and drug paraphernalia inside of Ethan’s bag and confiscate it. Peter gets to first class and sits down before Ethan arrives, having been upgraded to first class thanks to a faulty seatbelt. Peter is irritated to see him again and as the stewardess tells everyone to shut off their wireless devices, Peter and Ethan get into a fight because Peter wants to text his wife before the flight. Ethan tells him that phones are used by terrorists to ignite bombs which causes a minor panic. An air Marshall tells Peter to drop the phone and shoots him with a rubber bullet when he tries to respond.
Peter is kicked off of the flight along with Ethan. The legal advisor for the airline makes Peter sign a release and then informs him that he is on the do not fly list because of his actions. Peter is outraged and then discovers that he has lost his wallet. This means that without ID or money, he is stranded in Atlanta. While lounging about outside, he sees Ethan drive by and Ethan offers him a ride since he is going to drive to LA. Peter reluctantly accepts, hesitant to put up with Ethan and his dog, Sunny, for the 4 day drive.
They drive to a waffle house to eat and choose their path. Peter wants to hit the road immediately but Ethan wants to visit a craigslist drug dealer before they leave. Peter is pissed but Ethan reminds him that it’s his car and his money that they are using to get to Los Angeles.
The duo arrives at Heidi’s (Juliette Lewis) house. While Ethan tries out several different types of strains and showcases his acting abilities to Heidi and her boyfriend, Peter is left to deal with Heidi’s two children. Her son hits Peter until Peter loses it and punches him in the stomach. Ethan buys 200 dollars’ worth of marijuana and the two hit the road. When they arrive at the motel they planned on staying at, Peter finds out that Ethan’s card is useless since he maxed it out renting the car.
Peter goes to Western Union and asks Catherine to wire him 500 dollars in Ethan’s name. However, the clerk (Danny McBride) tells Peter that he cannot accept Ethan’s ID because Ethan’s real name is Ethan Chase, not Tremblay. They try and show him some head shots but the clerk won’t budge, instead clocking off early to go to Chili’s. Peter insults the clerk’s service in Iraq and he comes out and reveals that he is crippled. The clerk takes out a baton and beats Peter senseless.
After the fight, Peter is wiping off the blood and bitches Ethan out for wanting to go to Hollywood. Ethan acts out several scenarios poorly in order to showcase his abilities but he ends up crying over his dead father. He tells Peter that his father’s death is what brought him to George and shows him a Folgers can with his father’s ashes in it. Peter tells Ethan that his father abandoned him and Ethan laughs but then apologizes. The two decide to sleep in the car for the night.
Peter is awoken in the middle of the night and sees Ethan masturbating. He looks over and sees that Sunny is also masturbating and is grossed out, spending the rest of the night outside of the car. The next morning, when Ethan goes to shower, Peter throws all of Ethan’s stuff out and leaves him behind. Unfortunately, he sees that he forgot to leave Ethan’s father’s ashes and ends up turning around. Ethan is waiting dejectedly at the rest stop but Peter plays it off by bringing back donuts and coffee.
As the two continue driving, Peter goes to sleep. Unfortunately, Ethan starts to fall asleep and ends up driving the car over a bridge. The three are taken to the hospital and Peter calls his best friend, Darryl (Jamie Foxx), to pick him up and leave Ethan behind. Peter verbally abuses Ethan and then spits on Sunny. Darryl convinces Peter to bring Ethan alone because it would be bad Karma. They decide to bring him along but keep him in the back of Darryl’s truck so that they can bump him over speed bumps.
When they arrive at Darryl’s house, Darryl gives Peter the keys to his truck and some spending money, telling Peter that he’ll just get it all back on Friday when he visits Catherine and Peter for the birth of their child. Ethan is suspicious of Darryl’s intentions and blatantly asks if he has had sex with Catherine since photos of Catherine are all around the house. Peter is disgusted by the suggestion but slowly becomes suspicious. After Darryl makes coffee out of Ethan’s father, the duo leave and Peter makes an angry voicemail for Catherine, implying that he thinks they slept together.
Ethan drives while Peter naps from his medication and tokes up. When Peter wakes up, the windows are locked and the car is full of smoke. Peter wonders if he is high when his trip begins. The two bond while high, sharing stories and Buggles. Ethan tells him that he would have loved to spread his father’s ashes in the Mississippi and that he has been running a website dedicated to Two and a Half Men, his favorite show (it’srainingtwoandahalfmen.com) Unfortunately, Ethan takes a wrong turn and the two end up at the Mexican border. Ethan tells the guard that he has glaucoma, but when the guard turns around to check his ID, Ethan leaves Peter behind and runs away with Sunny.
The Mexican authorities tell Peter that he’s in serious trouble for attempting to bring drugs over the border and not having any identification. They mess with him for a few minutes before leaving him locked in a caravan. While the guards smoke Ethan’s weed, Ethan sneaks into the caravan park in a turban with Sunny. He hooks up a truck up to the caravan and drives off with Peter. The cops give chase but quickly end up crashing their cars.
After their harrowing escape, Peter is grateful for Ethan coming back for him and confesses that he was going to leave Ethan at the rest stop. Ethan tells him that it’s alright since Peter ended up coming back, showing what kind of person he really is. Peter tells Ethan to take the next exit and the two drive to the Grand Canyon. Peter tells Ethan that he should give his dad a grand gesture and scatter his ashes over the Canyon. Ethan gives a small eulogy before tossing the ashes into the wind. He then tells Peter that he has a confession of his own: He has had Peter’s wallet the entire time.
Peter is pissed and beats Ethan across the face when Catherine calls: She’s going into labor. Peter tells her that he will be there shortly and he forces Ethan back into the car. Ethan tells him that he’s sorry but that in the future, Peter should not punch him in the face since that’s how he’s going to make his living. Ethan looks for a bandaid but finds the border patrolman’s gun in the glovebox. He looks at it and then accidentally shoots Peter in the leg.
Peter, losing a lot of blood, asks Ethan to drive and wraps Ethan’s scarf around his leg. Ethan vomits from disgust and apologizes profusely but Peter makes it clear that he needs to drive them both to Catherine’s hospital. Ethan gets in the driver’s seat and speeds to the hospital.
The two arrive at the hospital and Peter hears Catherine screaming as she delivers the baby. He burst into the room only to see a black baby on the delivery cart. The African American family in the room tell him he’s in the wrong room and he goes to the next one to see Catherine in labor. He passes out from blood loss and Ethan introduces himself to Catherine as Peter’s best friend.
Peter and Ethan are looking at the babies in the nursery. Peter is sitting in a wheel chair and surprised that his son is actually a girl. Ethan tells her that she’s so rosy, she should be named Rosie. Peter doesn’t think it sounds right (Rosie Highmen), but just goes with it. Ethan tells him that he cut the umbilical cord while Peter was in surgery and Peter is surprised. He tells Ethan about his dream and Ethan laughs. Peter welcomes Ethan to L.A. and Ethan tells him they should go on another road trip and makes Peter promise to call him tomorrow.
A year later, Peter, Catherine and Rosie are in bed watching Two and a Half Men when Ethan calls. Peter tells Catherine that if “Ethan” is her first word, he will kill himself. Jon Cryer opens the door on the television and we see Ethan as a math tutor for the show. Charlie Sheen asks if Ethan has any pot and Ethan tells him he doesn’t for 9.50 an hour. Catherine tells Peter that he knows he loves Ethan and he hugs her, saying that they’ll call Ethan after the show finishes.