A group of friends gather for a pre-wedding party that descends into an existential nightmare when an estranged friend arrives with a mysterious game that awakens long-hidden secrets, desires and grudges.
Couple Shelby and Cyrus and their friends Reuben, Dennis, Nikki, Brooke, and Maya gather at Reuben's family home for a party before Reuben's wedding. Also due to attend is their mutual friend Forbes, whom they have not seen since a party at college for which Forbes got expelled for bringing his high school student sister Beatrice. Beatrice, obsessed with Dennis who was dating Nikki, was admitted to a mental facility.
Forbes arrives at the party, carrying a suitcase. Inside the case is a device that allows them to swap bodies with one another. Forbes explains that he and his colleagues would use it to play a game where they have to guess who is in their bodies. After doing a short demonstration on the group, they all eventually agree to participate, taking Polaroid photos for identification.
When round 1 begins, the group incorrectly guesses that Cyrus is in Dennis' body. To Cyrus' (who is actually in Reuben's body) surprise, Forbes, who is the one in Dennis' body, plays along with the lie, pretending to be Cyrus. Deciding to play along himself, Cyrus encounters Maya who is in Nikki's body, and the two kiss.
Unnerved by Forbes' lie, Cyrus declines to take part in round 2 but relents after the initially timid Shelby eagerly agrees to take part. Forbes explains this will be the last round before the machine takes 24 hours to recharge. When the round begins, Shelby swaps into Nikki's body and is quickly enamored by her good looks and successful social media career. Cyrus, who is in Forbes' body, panics and demands they end the game. As the group gathers in the house to switch back, the house's rooftop balcony collapses, and Reuben in Dennis' body and Brooke in Maya's body, who were up there having sex, fall to their deaths.
Arguing over what to do, Forbes refuses to let the group call the police and let the device go into their custody. Dennis, furious at the suggestion he has to swap into Reuben's body, argues with Cyrus and reveals Cyrus only settled for Shelby as he could not date Nikki. Dennis calls the police, claiming that Cyrus has killed Dennis and Maya. Forbes attempts to flee with the device but is subdued by Nikki. Shelby refuses to swap back and leave Nikki's body.
Cyrus tries to make amends with Shelby, but Maya tells Shelby that Cyrus had kissed her while pretending to be Forbes in the first round of the game. Forbes awakens, and Nikki tries to convince him to make the swap. Shelby appears, threatening to ruin Nikki's career with a damaging video. Shelby proposes that if Cyrus wants Nikki then she can stay in Nikki's body, and he can go into Reuben's body, while Dennis, in Cyrus' body, takes the fall for the deaths. Nikki allies with Forbes to swap back into their original bodies, and Nikki makes Shelby have an allergic reaction to peanut butter, refusing to give over the EpiPen unless Forbes checks the wiring. Dennis discovers money from his trust fund has been transferred into an offshore account and accuses Cyrus. The police enter the house as the group has set off the device.
The next day, Beatrice arrives at the house to discover the wedding has been called off. Seeing Dennis in Forbes' body, she reveals that she is the real Forbes, and that Beatrice had swapped bodies with him to enact revenge on Dennis and Nikki. In the aftermath, Shelby and an imprisoned Cyrus have returned to their bodies, while Beatrice, now in Nikki's body, escapes with Dennis' money and the device.
