Set in Wales during the 1980s, the story follows Oliver Tate, a 15-year-old introspective and precocious teenager. He has two main objectives: to lose his virginity before his next birthday and to extinguish the flame between his mother, Jill, and an ex-boyfriend, Graham, who has re-entered her life. Oliver monitors his parents' sex life by dimmer switch usage and begins a relationship with Jordana Bevan, a classmate with her own quirks and emotional intricacies.

As Oliver becomes increasingly involved with Jordana, navigating adolescent romance and trying to become a good boyfriend, he also observes the growing distance and coldness between his parents. Desperate to repair their relationship, Oliver concocts a plan to reignite his parents' passion for each other, while his relationship with Jordana grows more complex and demands more from him emotionally.