The film follows three friends who have been in a rut in their lives: Adam Yates (John Cusack) is dumped by his girlfriend; Nick Webber-Agnew (Craig Robinson) is a henpecked husband with a dead-end job at a dog spa; and Lou Dorchen (Rob Corddry) is a party animal in his 40s. When Lou is hospitalized for carbon monoxide poisoning, Adam and Nick sympathetically take him and Adam's shut-in 20-year-old nephew Jacob (Clark Duke) to a ski resort at Kodiak Valley, where the three had some good times in the past. During a night of heavy drinking in the hotel room's hot tub, they spill the contents of a drink called Chernobly on the console. The next day, they go skiing, but after too many strange occurrences (people dressed in 1980s fashion, music videos on MTV, and that Michael Jackson is still black), they realize they have traveled back to 1986. Not only that, but they have also assumed their younger bodies: they see each other as their normal age, but in their reflections and to other people, they appear as they did back then, except Jacob, who appears as himself but occasionally flickers.
A mysterious hot tub repairman (Chevy Chase) appears and warns them not to change anything as it might affect the machine. In order to minimize the butterfly effect, the guys plan to re-enact their experiences. Adam has to break up with his girlfriend Jenny (Lyndsy Fonseca) and get stabbed in the eye with a fork; Lou must pick a fight and get beaten up by Blaine (Sebastian Stan), a ski patrol bully; and Nick must have sex with a groupie, and give a bad performance with his band at an open mic event. Jacob discovers his mother Kelly (Collette Wolfe) is at the resort but is acting rather slutty; he tries to figure out who is his father.
The guys find the tasks rather difficult as Lou gets punched by Blaine and loses his backpack, but realizes he must face him again later at night, so he reluctantly challenges him again. Adam falls in love with Jenny again while meeting a music journalist named April (Lizzy Caplan) during the Poison concert. Nick worries about cheating on his wife even though the events occur before he even meets her. Later on, Lou tries to cash in on some sports betting using his knowledge of the game's outcomes; it works until he risks everything on predicting a game-winning touchdown, only to have the squirrel that he vomited on earlier at the resort crash the field and ruin the play.
Jenny turns the tables as she initiates the breakup with Adam, but Adam later meets April and they get along. Nick changes his destiny by covering the more upbeat "Jessie's Girl", followed by a "preview version" of "Let's Get It Started". When the repairman later informs Jacob that some nuclear chemical was the key to their time travel, Jacob realizes it was the "Chernobly", an illegal Russian energy drink. The guys rescue Lou, who was beaten up without his friends again, from falling off the rooftop. They go to Blaine's cabin to search for the drink, during which Lou finds and seduces Kelly. The guys realize that Lou is actually Jacob's father. After Lou finally punches Blaine, they retrieve the Chernobly and return to the hot tub where they create a vortex. Jacob and Nick get in the tub but Lou decides to stay in 1986, admitting he had indeed attempted suicide before. Adam volunteers to stay with Lou, but is thrown into the vortex.
Back at the present, Adam, Nick, and Jacob discover that Lou has changed history by founding "Lougle", and is enjoying a luxurious lifestyle with Kelly. Adam discovers that he is happily married to April, and Nick discovers he is a successful music producer married to a loyal and loving Courtney. The guys reunite at Lou's mansion with their families, satisfied with their new lives. During the film's closing credits, Lou is shown to be the frontman of "Motley L e" and sings in a video "Home Sweet Home".
Output: Why did the hot tub repairman tell the friends not to change anything?