Brian (Rick Hearst) falls under the control of a parasite named Aylmer that injects pleasure-inducing chemicals directly into his brain, but needs Brian to provide it with human brains in order to survive.
Brian tries to control his dependency, but the pain of withdrawal proves to be too much for him to handle. He warns his brother Mike (Gordon MacDonald) and girlfriend Barbara (Jennifer Lowry) to stay away from him because he can't control his behavior anymore and doesn't want them to get hurt, but Barbara doesn't listen. She follows Brian onto the subway to try to convince him to go into rehab (she and Mike are convinced that Brian's erratic behavior is caused by drugs), and ends up getting killed by Brian/Aylmer.
Brian returns to the alley behind his apartment building to hide his bloody clothing when he is attacked by his neighbors, an elderly couple whom Aylmer had escaped from and are desperate to get him back due to their own dependency issues. The husband snatches Aylmer just as he is injecting Brian's brain with the chemicals, making Aylmer accidentally inject too much juice into Brian's brain -- causing it to go haywire. Aylmer kills the wife and fatally injures the husband, who manages to squish Aylmer with his bare hands before dying. Brian, in agony from the electrical activity in his brain, staggers back to his apartment and locks himself in his room as the police arrive on the scene (another resident had called them after witnessing part of the struggle in the alley). He holds the elderly couple's gun up to his head and Mike, in the apartment, hears a gunshot. He and the police break the door down and find Brian -- still alive, still in agony, with a gaping hole in his head that has blinding light and electricity shooting out of it.
P.S. Duane Bradley (Kevin Van Hentenryck), the main character from director Frank Henenlotter's 1982 film "Basket Case", pops up in a cameo near the end. He's the creepy looking guy with the basket who briefly shares a subway car with Brian/Aylmer just before he kills Barbara.
Thanks Greg B!