Adam receives an unexpected call from his ex-girlfriend, Allison, who invites him to her wedding. Though hesitant at first, her insistence and the promise that she would attend his own wedding someday sway him into agreeing.
As Adam navigates through his day at work, he finds himself sitting at a conference meeting where his editing skills for documentaries are praised. His focus has been on a nature documentary, but as he grapples with his emotions, Claire suggests that he should step out more and enjoy some quality time with friends. They decide to go bowling, but Adam’s mood is so heavy that he barely participates, letting his bowling ball roll straight into the gutter.
Later, he attends dinner with his current girlfriend, Helen. In a surprising twist, Adam first proposes to her, only for him to abruptly change direction and break things off, leaving her furious and hurt. The reflective tunes of Tony Bennett’s "I Left My Heart in San Francisco" bring forth a wave of nostalgia, prompting Adam to recall his cherished moments with Allison through vivid flashbacks.
Determined to confront his past, Adam makes the journey to San Francisco for the wedding. Upon checking into a hotel, he encounters the overly enthusiastic Rohan, who wants to prepare for any guests Adam might have. Adam reassures him that he is alone, leading him to grapple with feelings of loneliness.
During a stroll through a bookstore, an unexpected reunion with Greg delights him, although Adam pretends not to recognize him. Greg fondly reminisces about Adam and Allison, claiming they were the perfect couple. This encounters sparks further flashbacks of how they met at college, where purple-haired Allison captivated him among the shelves of classic literature.
As the wedding draws near, Adam connects with Aaron, who is overjoyed to see him. Yet, Adam feels the weight of trying to explain who he is to others unaware of his past with Allison. After sharing an intimate conversation with her, he faces an awkward moment when a guest reveals his recent breakup, adding to the tension.
Seeking some comfort, Adam heads out for a jog and visits his mother, Deb. She is ecstatic to see him but is left heartbroken when Adam tears up a photograph of him and Allison, an emblem of their past. In a rather questionable tennis match with Aaron, Adam learns to keep his spirits alive despite not playing well.
Fate leads him to Allison's workplace, where they engage in an innocent conversation, revisiting their shared memories as they ride a cable car. Mark, his connection back home, raises questions about Adam attending the wedding solo, leading him to agree to bring along Talula, whom he meets and confides in before the celebration.
The wedding day arrives and Adam sits alongside Talula, anxiously waiting for the moment when everyone is asked to voice objections. In a moment of courage, he stands up, kisses Allison, to the cheers of the crowd. However, as he attempts to express his feelings, nervousness overtakes him, and he ends up choking on his words, leading to an awkward yet typical wedding conclusion.
At the reception, Adam converses with Allison's mother, Wendy, and later encounters an exuberant Aaron who announces a cake reveal and his culinary journey in Paris. After a brief respite outside, he shares a smoke with Federico, who offers unexpected but insightful advice laced with references to Tom Petty songs.
Back inside, Adam attempts to engage with the festivities, even as he finds himself hiding in the restroom, where there’s a poignant discussion with Allison amidst the chaos of the wedding. Deep down, they confront their breakup's unhealed facets, though the moment is fleeting as Leandra reminds Allison of her responsibilities.
As the evening winds down, Allison's father raises a toast, but in a fit of emotion, Adam makes a drastic move and knocks over the wedding cake, leading to an unexpected chase with Aaron and Allison. The night takes a wild turn when Adam is unexpectedly hit by a car but brushes it off, leaving Allison visibly concerned. Just as swiftly, he disappears into the night, catching a bus that takes him towards the unknown.
In the closing credit sequence, glimpses of Adam appear against breathtaking backdrops—mountainous landscapes in the cold, peaceful boat rides on a serene lake, and a solitary venture hitchhiking, signifying his journey through love, loss, and the road ahead.
