BRAIN DAMAGE

After suffering a brief illness, Brian experiences a powerful and comforting hallucination. He soon discovers that he has become host to a worm-like parasite that speaks perfect English and promises to give him a life free from worry. The hallucination was induced by a fluid injected from the parasite's mouth, through the back of Brian's neck and directly into his brain; the parasite offers Brian a steady supply of this fluid if "you'll take me for a walk." While under the influence of the "juice", Brian is incoherent and unaware of the world around him, allowing the parasite to kill and devour the brains of a night watchman in a junkyard, as well as a young woman at a nightclub. As Brian becomes addicted to the juice, he isolates himself from everything and everyone else in his life, which worries his girlfriend, Barbara, and his brother, Mike.

Brian awakens from his stupor long enough to learn about the murders. He is confronted in a courtyard outside his apartment building by his neighbor Morris, the previous owner of the parasite who dubbed it "The Aylmer", and who states that it has changed hands hundreds of times across the globe since the Middle Ages. Morris, who fed Aylmer with animal brains, warns Brian that feeding it humans will make it too strong to resist. Horrified, Brian rents a flophouse room to wean himself off the fluid and starve Aylmer, but Aylmer gleefully informs him that his body chemistry has irrevocably changed, and that the pain of withdrawal will be too much for him to bear. Brian soon relents, now consciously attending Aylmer as he hunts for victims.

Returning to his apartment, Brian discovers that Mike and Barbara have begun a relationship; realizing that he cannot control himself or choose Aylmer's targets, Brian tries to warn them away before fleeing to the streets. Barbara follows and confronts him on the subway, where Aylmer kills her. Back at the apartment, Morris and his wife, Martha, hold Brian at gunpoint to steal Aylmer back; Aylmer attacks them. As he feeds on their brains, Brian begs for another injection of juice. Aylmer agrees, which distracts them long enough for Morris to regain consciousness, grabbing and crushing Aylmer during the injection process. This kills Aylmer and forces an overdose, leaving Brian in agony. Screaming and bleeding juice, he runs to his room, puts Morris's gun to his own head and fires.

The police arrive at the apartment building. Joined by Mike, they break down Brian's door—finding Brian, who stares blankly with a large, glowing hole in his forehead, emanating with light and long crackles of electricity.

Brian, desperate for another dose of the parasite's fluid, is forced into an overdose when Aylmer is killed during an injection. In agony, Brian shoots himself in the head, leaving him alive but with a glowing, electrified hole in his forehead as police and his brother arrive.

The ending delivers a bleak culmination of Brian’s addiction: even after Aylmer’s death, the irreversible effects of dependency remain. The glowing hole and electrical crackles serve as a visual metaphor for the lasting damage caused by both the parasite and Brian’s choices. The final scene underscores the film’s darkly comic, tragic meditation on the inescapability and destructiveness of addiction.