Florence Marr, played by Greta Gerwig, is seen walking the Greenberg family's dog, Mahler. After picking up Phillip Greenberg's dry cleaning, she heads back to the family's home where they are busy preparing for a trip to Vietnam. Phillip (portrayed by Chris Messina) informs Florence that his brother, Roger, will be staying behind to watch over the house, asking her to assist Roger should he require help. Meanwhile, Carol, Phillip's wife (played by Susan Traylor), shares that Roger has just been released from the hospital after experiencing a nervous breakdown. As a carpenter, Roger has been contracted by Phillip to build a dog house for Mahler, though there are underlying doubts about his ability to complete the task.
When Roger, embodied by Ben Stiller, arrives, he calls Florence inquiring about some swimmers using the pool. She explains they are neighbors permitted to swim there. Florence later visits the house to collect her payment and feed Mahler, engaging in a brief and awkward conversation with Roger, during which she offers to run some errands for him. He hands her a short list that includes whiskey and ice cream sandwiches.
During a barbecue hosted by their mutual friend Eric Beller (played by Mark Duplass), Roger feels out of place, and Eric shows visible animosity towards him. At the gathering, Roger unexpectedly reunites with his ex-girlfriend, Beth (portrayed by Jennifer Jason Leigh), who agrees to meet him for drinks. Roger, disclosing that he is in Los Angeles to do nothing, faces Beth's assertion that it takes courage for a man of his age to lack ambition.
Upon inviting Florence for drinks, Roger, unfamiliar with driving as a New Yorker, relies on her to drive him. They stop at her apartment, where the mood turns intimate, and they begin to connect physically. However, after realizing her discomfort with casual relationships post-breakup, Florence halts their encounter.
Later, during a dinner with Eric, Roger gets grilled about his decision to reject a lucrative recording contract that their band was offered fifteen years prior, drawing ire from Eric who still grapples with their past. Roger emphasizes that their artistic integrity could not align with a commercial deal. As the days pass, Roger notices Mahler showing signs of illness and contacts Florence for assistance in taking the dog to the vet. They discover Mahler has an auto-immune condition, but this incident also serves to deepen the bond between Roger and Florence, with her developing strong feelings for him.
When Roger reconnects with Beth for drinks, the evening takes a turn for the awkward as she barely recalls their relationship, while Roger reminisces with fond precision. After attempting to ask her out, Beth declines, calling it a "terrible idea," and swiftly seeks the check.
Once Roger and Florence have an intimate encounter, a confrontation arises in which Roger criticizes her for returning to him, even as he expresses his reluctance to engage in a commitment. The turmoil escalates the following day when Roger reaches out to Florence, only to find her inebriated and preparing for an abortion the next day. Understanding the weight of the situation, Roger persuades her to allow him to accompany her to the clinic. However, since he doesn’t drive, his friend Ivan must transport them there.
After a tumultuous night at the Greenberg house, which includes a party hosted by Roger's niece, Sara (played by Brie Larson), and the arrival of Ivan, unresolved issues come to a head. An argument about the past recording contract surfaces, revealing Roger's guilt over having unwittingly sabotaged their future. The weight of their regrets and unmet expectations burdens both men, leading them to confront their unfulfilled dreams.
In a moment of vulnerability, Roger leaves Florence a heartfelt voicemail, articulating his feelings for her. When Sara invites Roger to join her trip to Australia, he eagerly considers the offer, but ultimately chooses to prioritize his feelings for Florence. He opts for an unexpected turn of events by picking her up from the hospital instead. The film concludes with Florence listening to Roger's heartfelt message, encapsulating the emotional journey they have shared.
