In 2010, three friends are dissatisfied with their lives. Adam (John Cusack) has been dumped by yet another girlfriend, and his video-game-obsessed nephew Jacob (Clark Duke) lives in his basement. Lou (Rob Corddry) is a party guy way past his prime who is divorced and his family wants nothing to do with him. Lou has also failed in several investments and is debt-ridden as a result. Nick (Craig Robinson) has a dead-end job at a dog spa and a controlling wife named Courtney (Kellee Stewart) who is allegedly having an affair.Lou almost dies from carbon monoxide poisoning in what his friends think is a suicide attempt. Adam and Nick impulsively take him and Jacob to the site of some of their most memorable weekends, the Kodiak Valley Ski Resort, "where no one got carded and everyone got laid". However, upon arrival they see that Kodiak Valley has fallen on the same hard times as the four men. Upon arriving, they encounter Phil, a one-armed bellman (Crispin Glover). During a crazy night of drinking in a hot tub, they spill a can of an illegal Russian energy drink called "Chernobly" (named in reference to Chernobyl, Ukraine) on the hot tub's controls. The next day, they wake up in 1986. They can see each other as their 2010 age and appearances, but in their reflections and to other people, they appear as they did in 1986. Jacob, who has not even been born yet, somehow is still a full-grown man as he was in his 2010 state. They arrive during "Winterfest '86," the weekend when Poison played to a huge crowd at the then-thriving Kodiak Resort. At first, the guys do not think that they have traveled back in time but are convinced when they find out that Michael Jackson is still black, and discover that Phil, the bellman has both arms.It is a time that was hard on the group. Adam broke up with his first girlfriend and got stabbed in the eye with a fork. Adam's sister, Kelly, conceived Jacob with an unknown father. Lou was beaten up by Blaine, the ski patrol bully, and none of his friends came to his aid. Nick's show ended in disaster with his band at an open mic contest. The four are at first concerned that if they change even the slightest thing in this time, it could have drastic consequences in the future (based on a tenuous understanding of the Ray Bradbury short story, "A Sound of Thunder"), they set out to do exactly what they did 24 years ago. Believing he must do everything the same as before, Nick has sex with a girl named Tara (Jessica Paré), but feels extremely guilty over being unfaithful to Courtney as his marriage to her occurred before the time travel. Later, they decide that this may be a chance to change their destinies. When Jacob begins to flicker in and out of existence, he warns the guys that if they continue what they're doing, then he may be wiped out of existence. A mysterious hot tub repairman (Chevy Chase) informs Jacob that the key to their time travel was the Chernobly, which contains chemicals that are vital to the time travel process. However, the Chernobly is in Lou's backpack, which was stolen by Blaine's gang.Nick calls Courtney (his future wife who is actually 9 years old when he calls) and yells at her about the evils of adultery. Courtney's father interrupts the conversation and Adam and Jacob stop Nick from telling him that Courtney is his bride-to-be. Adam's girlfriend, Jenny (Lyndsy Fonseca), dumps him before he can dump her (and he still gets stabbed by her, but in the forehead with a plastic fork), but he ends up spending the night with a music journalist, April (Lizzy Caplan). With Lou's encouragement, Nick & his band rock the crowd with performances of early-1980s hit "Jesse's Girl" and early-2000s hit "Let's Get It Started" - which he introduces as "a song from the future" that the audience gets to hear since this time (unlike last time) they were very receptive to his band's performance. Lou is beaten up by Blaine (Sebastian Stan), but finds the courage to punch him back, and all three of Lou's friends appear to confront Blaine's henchmen and recapture the Chernobly. Lou also seduces Kelly (Collette Wolfe) and conceives Jacob, solving the mystery of who Jacob's true father was. Also throughout the course of the time, they witness Phil almost get his arm taken off by a chainsaw and get stuck in the elevator shaft, but are disappointed when the arm doesn't become amputated, but as they are about to leave, Phil does lose his arm after being struck by a snowplow.The guys are set to go back to 2010, but, just as the hot tub starts to activate, Lou decides to remain in 1986, admitting to Adam that he was indeed trying to kill himself, and that if he goes back it will happen all over again. He says he wants to use his knowledge of future history to make investments, and do right by Kelly and Jacob. Adam says that if Lou is staying, so is he but Lou pushes Adam into the hot tub, just as it blasts the guys back into 2010. The gang arrives back in 2010 to find that Phil's arms are still in tact. Phil says that after the gang left, Blaine and his group managed to save his severed arm by putting it on ice and taking him to a nearby hospital to have it re-attached.Adam, Nick, and Jacob discover that Lou, Kelly, and Jacob are now a happy family enjoying a lavish lifestyle. Lou has become a successful rock star with his favorite band, "Mötley Loue" and is the founder of an Internet search engine called "Lougle". Adam discovers that he eventually married April, and Nick is a successful music producer married to a loyal and supportive Courtney. She tells him about a wrong number phone call she got when she was 9, that was enough to keep her from trying to cheat on anybody. Adam, Nick, Lou, and Jacob reunite at Lou's mansion with their families, satisfied with their new lives.