In San Fernando Valley, 1973, a group of boys eagerly awaits their moment in front of the camera while a couple of their peers mischievously detonate a cherry bomb in the bathroom. Among them, Gary Valentine (Cooper Hoffman) finds himself drawn to the photographer's assistant, Alana Kane (Alana Haim). Despite their age difference—15 and 25—Gary's charm manages to persuade Alana to spend time with him, albeit not on a romantic date.
The duo meets at a restaurant familiar to Gary, where they engage in conversation about their respective careers. Gary shares that he works with his mother, Momma Anita (Mary Elizabeth Ellis), at a public relations firm. Promising that they won’t forget each other, they take a stroll home, giving a glimpse into their lives: Alana resides with her parents and two sisters, while Gary lives with his brother, Greg (Milo Herschlag). Their professional dynamic shines through when Gary and Anita discuss their partnership with restaurant owner Jerry Frick (John Michael Higgins) about launching a new Japanese eatery. Jerry’s treatment of his wife, Mioko (Yumi Mizui), soon becomes evident through his off-putting accents.
Unexpectedly, when Anita cannot accompany Gary to a New York trip for auditions, Alana steps in. This leads to Gary reconnecting with his co-stars from "Under One Roof," featuring the talented Lucille Dolittle (Christine Ebersole). His comedic antics grow more outrageous, culminating in a playful encounter that leads to a reprimand from Lucille. Meanwhile, Alana becomes acquainted with another actor, Lance (Skyler Gisondo), who enters the picture upon their return to California.
Their relationship soon faces strain when Alana's Jewish upbringing clashes with Lance’s atheism during a family dinner. This dissonance ultimately pushes Alana to end things with him. Gary pivots, diving into the waterbed business alongside Alana and friends Kirk (Will Arangola) and Mark (Griff Giacchino). Together, they attend the "Soggy Bottom" expo, where the night takes a wild turn as Gary finds himself in legal trouble, only to be supported by Alana.
With Alana managing the business phone calls, Gary's affection for her grows, despite their playful bickering regarding her potential nudity in films. An event where she unexpectedly kisses another guy out of jealousy showcases the complexity of their relationship. Alana's pursuit of her ambitions leads her to audition for a role with Jack Holden (Sean Penn), who invites her into a whirlwind of Hollywood glamor, making her an object of admiration and envy.
A series of mishaps ensues, including an awkward delivery to Jon Peters (Bradley Cooper), which involves both peril and comedy. As Gary and Alana navigate their respective career hiccups—his waterbed business hit hard by the gas crisis and her political volunteer work—they find themselves drifting apart. However, in a twist of fate, they eventually cross paths at a movie theater, igniting the spark between them once more.
In a heartfelt reunion, Gary brings Alana back to his arcade, introducing her affectionately as "Mrs. Alana Valentine," prompting a sweet, spontaneous kiss and reestablishing their bond. After a series of ups and downs, they declare their feelings, with Alana finally expressing her love for Gary, cementing their connection amidst the chaos of their youth.
