Eric Keppler, portrayed by Jason Sudeikis, is a thirty-something man who embodies the spirit of an eternal party lover. Living in his father's spacious house in the Hamptons, he frequently hosts extravagant themed parties with his eclectic group of friends. One of the most notable events is the "White Trash Bash" that takes place on July 3, where familiar faces gather, including his ex-girlfriend Sue Plummer, played by Michelle Borth, the nerdy workaholic Adam Richman, embodied by Nick Kroll, the laid-back Mike McCrubber, portrayed by Tyler Labine, the reserved teacher Laura LaCarubba, featured by Lindsay Sloane, the fun-loving Kate played by Lucy Punch, along with her fiancé Glenn, portrayed by Will Forte, and couples therapist Alison Lobel, portrayed by Lake Bell. The guest list also includes law student Doug Duquez, played by Martin Starr, his artistic girlfriend Willow, portrayed by Angela Sarafyan, and Alison's boyfriend Marcus, played by Rhys Coiro. During the festivities, Eric crosses paths with Kelly, played by Leslie Bibb, who seems to be pregnant.
The very next day, Eric's father, Jerry, portrayed by Don Johnson, arrives with news that he intends to sell the house. When the friend group returns the following weekend, they discover it is indeed on the market, being handled by real estate agents Dody, played by Lin Shaye, and Kelly. In a bid for one last hurrah, Eric and Mike decide to organize the ultimate party for Labor Day weekend. Reflecting on the younger generation's more relaxed attitudes toward sex, Eric proposes holding a small orgy among their friends. Initially met with hesitation, the group gradually warms to the idea as Mike and Eric argue that this could be their final opportunity to bond before life pulls them apart. Sue comes forward, admitting her lingering feelings for Eric from high school, yet Eric finds himself drawn to Kelly in a misguided effort to impede the house's sale. As the dynamics evolve, Doug and Willow join the plan, encouraging Doug to bolster his confidence.
At Kate and Glenn's wedding that follows, a drunken Adam inadvertently reveals his job loss, triggering his agreement to partake in the orgy out of frustration. The ensuing chaos leads to Glenn discovering their plans, causing rifts within the group. A couple of weeks before the weekend, Eric and Mike visit an underground sex venue disguised as a furniture store named ‘Fred's Beds’ for insights on orgy participation, collecting tips from Mike’s uncle Vic, humorously played by David Koechner.
As Labor Day approaches, Eric finds himself conflicted over his feelings for Kelly and doubts about the orgy. In a turn of events, Dody agrees to delay the house sale, allowing time for one final party with friends. However, just before the big day, some guests drop out, igniting an argument that threatens to derail their plans. In this turmoil, Sue's feelings for Eric inadvertently surface, and much to their surprise, Glenn and Kate crash the party intending to join the orgy but end up distracted in their own romantic escapade.
When the day finally arrives, the group struggles with their inhibitions for a bit before ultimately reconciling and diving into what they have planned. Through spirited interactions and unabashed fun, the night unfolds with numerous passionate escapades around the house, drowning their insecurities in alcohol and revelry. The jovial atmosphere leads everyone to let loose, with Eric indulging in a connection with Sue, Mike pairing with Alison, Doug finding solace with Laura, and Adam also getting involved.
As the sun rises after the night's debauchery, the aftermath leaves everyone nursing hangovers while Mike provides humorous mementos from their adventurous night. Doug's newfound confidence shines through as he gifts Mike his finished album, with waves of fresh beginnings prompting each character to explore new relationships. By the end, Sue accepts her feelings for Eric, Alison and Marcus reconcile, and new romances blossom between Adam and Laura, leaving Eric to reunite with Kelly on the beach, where they learn that the house sale has hit a snag. Enthusiastic about new possibilities, Eric starts planning yet another gathering for Memorial Day.
