Three estranged and downhearted friends—Adam Yates, who recently experienced a harsh breakup with his girlfriend (taking everything he owned and burning the remainder); Nick Webber-Agnew, a husband who feels neglected and has taken on his wife's last name while toiling away at a dead-end pet store job; and Lou Dorchen, an unemployed, hard-drinking slacker in his forties—find themselves reuniting in 2010. This reconnection is prompted by Lou's hospitalization due to carbon monoxide poisoning, which the doctor suspects may have been a suicide attempt. With Lou having no family and a string of failed business ventures, his friends are asked to keep an eye on him.
To lift Lou’s spirits, Adam and Nick invite him to join them along with Adam's slacker nephew Jacob (Clark Duke), who lives in Adam's basement engrossed in video games, for a nostalgic trip to the Kodiak Valley Ski Resort, a place they frequented during their youth. However, upon arrival, they’re disheartened to find many shops boarded up and their hotel in disrepair.
During one wild night of excessive drinking in their hotel’s hot tub, the friends pour a questionable Russian energy drink called "Chernobly" into the mix. The unexpected aftermath leads to a morning where Lou hilariously vomits on a squirrel. As they hit the slopes, they’re shocked to discover they’ve somehow traveled back to 1986. To their astonishment, Adam, Lou, and Nick find themselves inhabiting their youthful bodies, while Jacob appears unchanged since he wasn’t yet born.
A mysterious hot tub repairman (Chevy Chase) appears and issues a stark warning: they must avoid altering history to prevent dire consequences. Determined to minimize the so-called butterfly effect, the group resolves to replicate their past experiences. This includes Adam needing to break up with his girlfriend Jenny (Lyndsy Fonseca) by getting stabbed in the eye with a fork, Lou needing to confront a ski patrol bully named Blaine (Sebastian Stan) and sleep with a random girl, and Nick set to have a fling with a groupie named Tara (Jessica Paré) and perform "Careless Whisper" with his band during open mic night.
As the friends embark on these challenging tasks, they encounter their past in peculiar ways. Lou initially suffers at the hands of Blaine but later bravely seeks him out for further confrontation. Meanwhile, Adam's spark for Jenny reignites, complicating his intentions of their breakup when he finds himself captivated by a free-spirited music journalist named April (Lizzy Caplan) during a rock concert. Nick grapples with the morality of cheating on his wife, even as these events transpire before they even meet.
Tragedy strikes when Lou’s plans to take advantage of his foreknowledge of a football game go awry, leading to chaos involving the squirrel he had previously vomited on. In a turn of events, Jenny takes control of the situation and initiates their breakup, leading to further misadventures for Adam that involve a painful fork incident. Lou, enraged by Adam’s change of heart regarding Jenny, endeavours to manipulate events for personal advantage.
As the plot thickens, each character struggles to fulfill their intended roles while navigating nostalgia and their deeper emotions. Nick finds empowerment through music, culminating in an uplifting performance that highlights a pivotal moment in his journey. The revelation of the Chernobly's role as the catalyst for their time travel sparks a sense of urgency.
Their escapades culminate in a tense effort to retrieve the Chernobly from Blaine’s cabin, where Lou unwittingly finds himself in a romantic entanglement with Jacob’s drug-addicted mother, Kelly (Collette Wolfe). The stakes escalate when Jacob mysteriously disappears, revealing Lou’s connection as Jacob’s father, complicating their dynamics further.
In an emotional climax, Lou, having found new purpose, resolves to stay in 1986 to forge a meaningful future while Adam insists on joining him. Their friendship culminates in a bittersweet parting as Lou thrusts Adam back into the vortex, returning to the present.
When they awaken, Adam, Nick, and Jacob marvel at the new reality: Lou has become a prosperous mogul with the creation of “Lougle,” leading a lavish life with Kelly. Adam is happily married to April, and Nick enjoys his success as a music producer with a supportive partner by his side. The reunited friends come together at Lou's mansion, celebrating their newfound happiness and lives enriched by their shared past and present.
