Will Henry, a graphic novelist and art instructor, is busy putting up decorations for the birthday celebration of his five-year-old twin daughters, Clio and Colette. During his preparations, he unexpectedly discovers his long-time partner, Charlie, with another man, Gary, a monologist with whom she is having an affair. Charlie candidly reveals her unhappiness and her decision to leave Will for Gary.
Fast forward a year, Will is now living in a smaller space, seeing his daughters on the weekends. Although he enjoys a strong bond with them, Charlie is skeptical about his ability to be a responsible parent. To complicate matters, she informs him that she is pregnant and getting married to Gary.
In his art class, Kat, one of Will's students, invites him to dinner with her single mother, Diane. Will finds Diane charming and appealing, but she degrades Kat's studies and dislikes Will's career path. After a slightly awkward evening, he learns that Diane is unfortunately already in a relationship.
One evening, Charlie drops the twins off at Will's place unexpectedly, seeking his help as their nanny has quit. Though he initially struggles with the responsibility, he quickly adapts. However, the day takes an unexpected turn when a bomb threat cancels school, prompting Will to ask Kat to babysit while he teaches his class.
When Will returns to pick up the girls, Diane criticizes him for entrusting Kat with his daughters while she has her own educational commitments. Since the girls are asleep, Will ends up staying the night. As their conversation deepens, Diane gains an appreciation for graphic novels and confesses she has broken up with her boyfriend. They share a kiss, but the moment is interrupted when one of the twins comes calling for him. The following morning, Kat asks Will to read her graphic novel.
As Will’s relationship with Diane becomes more casual, tension mounts between him and Charlie, each blaming the other for the confusion affecting the girls. One night, Charlie calls in desperation, claiming there's an emergency; when Will rushes over, he finds that the twins have locked themselves in the bathroom, expressing a desire to live with him. Upset, Charlie confides her fears about Gary's commitment and they share a kiss. The girls emerge, and the three head out for ice cream, with Charlie assuring Will they’ll discuss their relationship later.
After revealing the kiss to Diane, she confronts Will about his lingering feelings for Charlie. Not knowing the answer, Will struggles with the idea of moving on, while Diane tells him that, in time, feelings of hurt will fade. She ultimately decides to end their relationship.
Will meets Charlie and the twins in the park, optimistic about their situation, only to learn that she's planning to marry Gary again. Enraged, Will leaves and channels his frustration into creating vengeful artwork against Gary. He comes across Kat’s graphic novel, titled "Mother Fuckers," which illustrates her mother’s painful journey through dating as a single mom, depicting Diane's escalating disappointment with each breakup.
Will later encounters Kat, who is furious with him for hurting her mother. In turn, he shares his own art with her, showcasing his emotional turmoil regarding Charlie. Soon after, Kat returns his work, urging him to let Charlie go. She shows him a page depicting Diane and advises him to reconnect with her.
On the day of Charlie and Gary’s wedding, Will arrives only to find Charlie missing. After confronting Gary and throwing a punch (despite acknowledging Gary's affection for Charlie), Will finally locates her. She expresses her fear of making another mistake, but he reassures her that their relationship wasn't a mistake; it simply had its time. They agree to remain friends, and Charlie proceeds with her wedding to Gary. Will departs alone, having pilfered some flowers from the celebration, likely destined to visit Diane once more.
