Jamie, portrayed by Catherine Keener, breaks the news to her ex-husband, John C. Reilly, that she is set to marry someone else. Despite being apart for seven years, this revelation leaves John emotionally shattered and deepens his already existing depression. At a party the following night, John, overwhelmed by drink, finds himself in a compromising situation when he ends up urinating in the bushes. Here, he encounters Molly, played by Marisa Tomei, who strikes up a chat with him. After an unexpected turn of events, Molly spends the night at John's place but decides to leave shortly after they have sex.
The following evening, their connection sparks once again when Molly returns for dinner, only to leave after another intimate encounter. Curious about her, John follows her home and ends up dozing off in his car outside. The next day, as he approaches the house, he meets Molly's son, Cyrus, who is only twenty-one. Cyrus is cordial and invites John in, creating an uneasy atmosphere for John when he sees the unusually close bond between Molly and her son.
As the days pass, John experiences growing anxiety, particularly when he cannot find his shoes he left in Molly's living room. His unease escalates as he suspects Cyrus of playing mind games with him. To ease his worries, John convinces Jamie to meet Molly and Cyrus, but she finds Cyrus sweet, albeit too close for comfort with his mother. Despite feeling relieved, John's return to Molly's home takes a turn when their romantic moment is interrupted by Cyrus’s screams in the middle of the night, prompting Molly to rush to comfort him and leaving John alone.
When John tries to express his feelings to Molly, Cyrus interferes under the pretense of a night terror and advises John to back away from the relationship, stirring further doubt in John’s mind. After writing Molly a note, he heads home. The following day, Cyrus manipulates the situation by telling Molly that John thinks she is too pushy, leading to a significant argument that leaves Molly distraught. Eventually, after some time apart, Cyrus believes he can no longer stay with his mother and moves out.
Feeling elated, John contemplates moving in with Molly, but their romantic encounter is again disrupted by Cyrus, who fabricates a panic attack to break them apart. John confronts Cyrus privately, leading to the revelation that Cyrus has been intentionally sabotaging their relationship. As tensions rise, the night before Jamie’s wedding, John warns Cyrus not to stir up trouble, but at the wedding, Cyrus, feeling threatened, accuses John of trying to steal his mother away. Their confrontation turns chaotic, attracting attention and forcing John to deny the accusations.
Despite Molly's support of John's explanation, he ultimately decides to end their relationship, fearing that Cyrus will always be a hurdle and he will find himself alone eventually. He opts for a cheaper living situation to distance himself. These events compel Molly to confront Cyrus about their unhealthy relationship. After some soul-searching, Cyrus attempts to mend fences and reconciles with John, leading both to embrace their relationships anew, hoping for a healthier future together.
