Sixteen-year-old Tess DeNunzio (Sadie Sink) lives in Pittsburgh with her mother Elly (Jessica Capshaw), stepfather David (Justin Bartha), and younger half-sister Em (Vivien Lyra Blair). Tess's biological father Nick (Theo Rossi), a German Shepherd breeder, lives nearby in a poorer neighborhood in Braddock. After Tess's three-year-old half-sister, Zoe, died in a hit-and-run accident on the day of the September 11 attacks while Tess was supposed to be watching her, Tess helped David care for Em while Elly coped with depression. The family attends therapy together, though Tess feels excluded from David and Elly's grief. Tess has a breakdown after discovering of Elly's emotional affair with Justin, a much younger grocery store clerk. Tess moves in with Nick, though she regrets abandoning Em. Tess begins to care for one of Nick's puppies. Nick agrees to drive Em home from school so that the sisters can spend time together. Tess and Nick attend church together, but she runs from the church screaming when a hymn reminds her of 9/11. Tess is attracted to her neighbor, Jimmy Freeze (Kweku Collins), who claims to know her father. She offers him a beer, but Nick intervenes and instructs her not to socialize with him. When a suspicious man approaches Tess on the porch with a wad of cash, Tess realizes that Nick is dealing drugs out of the back of his truck. Tess confronts her father, but he doesn't stop selling drugs. One night, Jimmy climbs through Tess's window. While smoking weed together, Jimmy reveals he struggles to bond with his father and stepmother, and was sent to a correctional facility. He also discusses his mother's death, though Tess doesn't mention Zoe. They kiss goodnight. Jimmy continues to visit her at night, but Tess keeps their developing relationship a secret from Nick. When summer begins, Em feels isolated, as she only sees Tess on rides home from school. Tess and Elly begin to have weekly phone calls, during which Tess notes that they're both improving and Elly admits she's afraid of losing Tess, which Tess promises won't happen. Though missing Em and Elly, Tess continues to live with Nick, finding it easier to forget Zoe there. Jimmy gets Tess a summer job at Kennywood, an amusement park where he works. Soon, Jimmy and Tess profess their love for one another. On the evening of Tess's 17th birthday, Nick and Jimmy are both arrested; Tess realizes Jimmy sells drugs with her father. Her mother brings her presents; Tess tells her about the park and Jimmy. When Nick and Jimmy arrive home, Jimmy stays with Tess. They have sex for the first time and Tess tells Jimmy about Zoe: Tess missed the school bus, and her mother instructed her to watch Zoe outside. Tess ran inside to watch news coverage of 9/11, and returned to find Zoe stepping off the curb, only to be hit by a car. Jimmy assures Tess that he still loves her. Tess moves back to her old house. Tess stands on the grass where Zoe landed after the car accident and cries. She reconciles with her mom, David and Em.