In 1979, 15-year-old Jamie Fields resides in sunny Santa Barbara with his 55-year-old single mother, Dorothea. They share their home with two tenants: Abbie Porter, a 24-year-old photographer battling cervical cancer, and William, a carpenter and mechanic. Jamie's closest companion is 17-year-old Julie Hamlin, who frequently stays over but is adamant about remaining friends, believing that taking their relationship into a sexual realm would jeopardize their bond.
Feeling a sense of disconnect with her son, Dorothea seeks the assistance of Abbie and Julie in raising Jamie. In response to this, Jamie impulsively escapes to Los Angeles with friends to catch a rock concert. Upon his return, Julie confides in him about her unprotected sexual encounter and the anxiety surrounding a potential pregnancy. Accompanying Abbie to a doctor's visit, Jamie learns that she is cancer-free but might never be able to have children. While at the clinic, he picks up a magazine article on home pregnancy tests, which ultimately leads him to purchase one for Julie; fortunately, it returns a negative result. As a gesture of gratitude for his support, Abbie crafts a heartfelt mixtape for Jamie and reveals intimate details of her life, including how her delayed return from New York followed her diagnosis. Tragically, her mother could not face the truth that the cancer was caused by the fertility drug DES while Abbie was in utero, leading to her move with Dorothea.
One day, after witnessing Julie sneak out of Jamie's window, Dorothea engages her in conversation, which unfolds into a candid dialogue about Dorothea's lack of real relationships since Jamie's father departed years earlier. Inspired by Jamie's struggles, Dorothea persuades Abbie to introduce her to "the modern world." They head to a punk club, where William kisses Dorothea; however, she turns him down when she learns of his affair with Abbie. In the aftermath, Abbie ends up in a heated altercation, while William ultimately decides he no longer wishes to be with her. Seeking wisdom, Abbie visits Jamie, only to discover him in bed with Julie. She admonishes the couple, warning them not to confine themselves to the limits of Santa Barbara.
Later, Jamie pleads with Abbie to take him back to the club, where the night spirals as he drinks too much and unexpectedly kisses another woman. Meanwhile, Dorothea takes on the role of a guide for William, teaching him the nuances of pursuing long-lasting relationships instead of fleeting encounters. When Abbie shares the details of Jamie's escapade with Dorothea, she doesn't react with anger; instead, she feels a tinge of nostalgia about the kind of experiences she can no longer witness firsthand as his mother.
Abbie introduces Jamie to feminist literature, which piques his interest, but Dorothea deems the material too advanced for him, resulting in a confrontation with Abbie. As tensions rise during a dinner party, Abbie reveals her exhaustion from menstruating, leading to an amusing moment where she makes the men at the table state the word "menstruation." This candid comment incites Julie to share her own experiences with her first period and sexual encounter, effectively derailing the evening.
Feeling frustrated, Jamie informs Julie that he doesn't want her to spend the night if the intent is merely to talk. Stung by his words, she proposes a spontaneous road trip up the coast. At a hotel during their getaway, Jamie professes his love for Julie, but she argues that their closeness makes intimate relations complicated. Their disagreement escalates as Julie accuses him of being just like "the other guys." In a fit, Jamie storms out, and Julie reaches out to Dorothea, but by the time she arrives with Abbie and William, Jamie has already returned. Dorothea shares with Jamie her intent behind asking for help from Abbie and Julie; she hopes for Jamie's happiness beyond her own experiences. This heartfelt moment mends their relationship, allowing them to return home to Santa Barbara, where Dorothea finally opens up about her aspirations and emotions.
In a reflective epilogue, we learn that Julie will start birth control, attend NYU, drift away from Jamie and Dorothea, find love, move to Paris, and ultimately decide against having children. Abbie chooses to remain in Santa Barbara, marry, set up a photography studio in her garage, and successfully welcome two sons into her life. William will continue living with Dorothea for another year before embarking on his own journey to Sedona to open a pottery shop and navigate his own romantic ventures. Dorothea finds companionship after 1983, living alongside a man until succumbing to metastatic lung cancer in 1999. Years later, as an adult, Jamie marries and becomes a father, yet struggles to convey the essence of Dorothea to his son.
