Donnie's father, Eddie (Holmes Osborne), takes Donnie to his therapist, Dr. Lillian Thurman (Katharine Ross), and nearly runs over Roberta Sparrow (Patience Cleveland), a seemingly senile old woman known as "Grandma Death". Afterward, Dr. Thurman increases Donnie's medication and begins hypnotherapy. Frank continues to appear to Donnie and manipulates him to commit a series of crimes and, also, tells Donnie about time travel. Donnie floods the school, steals his father's gun, and burns the home of Jim Cunningham, where firemen uncover a "kiddie porn dungeon".
Donnie, along with his older sister, Elizabeth (Maggie Gyllenhaal), decide to throw a Halloween party while their mother, Rose (Mary McDonnell), and younger sister, Sam, (Daveigh Chase), are away at a dance competition. Gretchen comes to Donnie's house for safety because her mother has suddenly disappeared, likely because of her threatening stepfather, and she and Donnie have sex for the first time at the party. At midnight, Donnie realizes that the 28 days have passed and that only 6 hours remain until the end of the world. Donnie leaves to visit "Grandma Death" along with Gretchen and two friends. While there, they are assaulted by the high school bullies (Alex Greenwald and Seth Rogen). Gretchen is knocked unconscious and thrown into the street. An approaching car swerves to avoid "Grandma Death", standing in the road, but runs over Gretchen, killing her. As the bullies run off, a man wearing a rabbit costume emerges from the car, and it's Elizabeth's boyfriend, Frank. Frank starts yelling at Donnie, who shoots Frank with his father's stolen pistol.
Donnie carries Gretchen's lifeless body to his home, places her in the family car, and speeds away, watching from nearby as a tornado forms over the city. For once, Donnie seems at peace as a vortex engulfs the jet his mother and sister are returning home in. The storm damages the plane causing an engine to fall off before Donnie transports it back in time to 28 days earlier using a wormhole. This time, though, Donnie chooses to stay in bed. He laughs and turns over as if to sleep just as the jet engine crashes through his bedroom, causing a plank to impale him (on the DVD's outtakes