In 2002, Elaine "Lainey" Dalton (@Alison Brie) creates a bit of chaos in a college dorm, leading to a pivotal moment when Jake Harper (@Jason Sudeikis) steps in to save her from being kicked out by campus security. He finds out that Lainey's mission was to lose her virginity to her teaching assistant, Matt (Adam Scott), whom Jake dismisses as the dullest individual he's ever met. As the story unfolds, Lainey discovers that Jake is also a virgin, which sets the stage for an unexpected connection between them.
Fast forward to the present, Lainey finds herself breaking up with her long-term boyfriend, Sam (Adam Brody), after confessing to cheating on him not once, but 16 times. Their relationship has become strained, with Sam often interrupting her and doubting her fidelity, including accusations of infidelity with his brother, which Lainey vehemently denies. Although Lainey is working as a kindergarten teacher, she had always aspired to attend medical school.
In search of guidance, Lainey eventually attends a meeting for love addicts where she unexpectedly runs into Jake, who is struggling with commitment issues after being dumped by his girlfriend Hannah (Margarita Levieva) due to his own infidelities. As she contemplates her complicated relationship with Matt—the same man she’s been involved with through it all, now engaged to Emma (Katherine Waterston)—Lainey finds herself entangled in yet another romantic dilemma.
Encouraged by her friend Kara (Natasha Lyonne), Lainey reaches out to Jake, who has recently sold his online advertising business to Paula (Amanda Peet). Jake's interest in Paula is complicated by her being a divorced mother, and though he pursues her, she remains evasive.
During a double date with Jake and his business partner Xander (Jason Mantzoukas), Lainey and Jake open up about their respective sexual struggles. Jake expresses his desire to be intimate with Lainey, but she proposes that they remain friends, establishing a “code-word”—“mousetrap”—to navigate the tension. As they spend more time together, they forge a deep friendship, with Lainey confiding about her ongoing feelings for Matt. Meanwhile, Jake helps her understand the emotional dynamics that come into play in her relationships.
As their bond strengthens, their friends start to assume they’re dating, further confusing their feelings. Jake discovers that Lainey hasn't experienced an orgasm in over a year and takes it upon himself to teach her the importance of self-exploration. However, the more time they spend together, the more Jake wrestles with his feelings for Lainey, especially when he learns of her plans to re-enroll in medical school in Michigan.
Despite their undeniable connection, Lainey begins dating Chris Smith (Marc Blucas), which puzzles Jake, who is trying to explore a romantic path with Paula. Following a date with Chris, Lainey unexpectedly encounters Matt and his pregnant wife, fracturing her composure. Simultaneously, Jake's night with Paula takes a chaotic turn when he calls out Lainey’s name, leading to introspection for both.
They share a night filled with realizations, recognizing their love for each other but still feeling the weight of past traumas—Lainey especially, still harboring pain from Matt. When Lainey moves to Michigan, Jake tries to move on, only to punch Matt in the face two months later after seeing him during brunch, leading to his break-up with Paula and subsequent arrest. In a moment of vulnerability from the police station, Jake confesses his feelings for Lainey, who reciprocates his love.
Soon after, a legal battle ensues as Matt decides to sue Jake for emotional distress, refusing all settlement offers. Lainey confronts Matt, leveraging the secret of their past against him to ensure Jake's settlement is accepted. Upon leaving Matt's office, Lainey reunites with Jake, and they head off to tie the knot, leaving behind the tumultuous journeys that ultimately led them back to one another.
