In 1973, two families residing in an affluent Connecticut suburb find themselves embroiled in a web of adultery and substance abuse during one tumultuous Thanksgiving weekend. The first family, the Hoods, is made up of parents Ben and Elena, along with their children Paul and Wendy. Feeling trapped by the monotony of his marriage and career, Ben seeks solace in an affair with Janey Carver, who is married to their neighbor Jim. Contrarily, Elena is also restless and yearns to add a spark to her life, although she remains uncertain about how to achieve that.
Meanwhile, Wendy Hood becomes involved in flirtatious escapades with the Carver boys, while her brother Paul is infatuated with Libbets Casey, a classmate from his boarding school. Tension builds when Ben finds Mikey Carver peculiar and withdrawn, contrasting with the adventurous, reckless behavior exhibited by his younger brother Sandy, who delights in dangerous toys.
The simmering tensions erupt when Janey neglects to meet Ben, prompting him to lash out at her. As Thanksgiving winds down, the Hoods face a confrontation when Elena uncovers Ben's infidelity. Despite this rift, the couple attends a neighborhood party, a gathering that soon takes a shocking turn as it evolves into a "key party" where couples swap partners by drawing keys from a bowl.
As the night unfolds, Ben indulges in excessive drinking, leading to an escalating sense of discomfort. His agitation peaks when Janey picks the keys of a younger man, causing him to retreat to the bathroom in shame. Outside, a treacherous winter storm intensifies, adding a layer of chaos to the evening.
In the midst of the party, as couples pair off, only Jim and Elena remain. Their brief encounter in Jim's car leads to a moment of regret, prompting him to drive her home. In another part of town, Wendy attempts to rendezvous with Mikey but instead finds Sandy, whose feelings for her lead them to a night of unintended intimacy.
Meanwhile, Paul heads to Libbets’ Manhattan apartment, only to find Francis also invited, which turns his night into a bitter disappointment. Through laughter, drinks, and unwelcome truths, Libbets reveals the boundaries of their relationship, thrusting Paul into an emotional upheaval, compelling him to return home in haste.
The unfolding tragedy reaches a climax when Mikey, out in the storm, succumbs to an unfortunate accident involving a fallen tree. The news shocks both families, culminating in a somber gathering as Ben discovers Mikey's lifeless body. Emotions run high as grief unites the Hoods and Carvers, leading them to confront their sorrows and secrets.
At dawn, Elena finds Wendy in bed with Sandy and snaps her into reality, while Janey remains blissfully unaware of the night's disastrous events. Family dynamics shift, and the harsh reality of loss brings them all together as they deal with the aftermath of the weekend’s choices. Ben, heartbroken and overwhelmed, grapples with the weight of his behaviors, while his family stands by him, sharing a moment of collective sorrow.
