In 1985, Ash, the leader of a street gang in New York City's Lower East Side, receives a sample of a drug from a mortally wounded man. Before dying, the man leaves him with the words: "Find Banana Fish..." The phrase catches Ash's attention because it is the same mysterious term occasionally spoken by his older brother Griffin, who lost his sanity after serving in the Vietnam War. Meanwhile, the man had been pursued by Dino Golzine, a powerful figure in the criminal underworld. As the last person known to have contacted him, Ash falls under suspicion. When Eiji, a Japanese photographer's assistant who meets Ash during a magazine interview, becomes caught up in the mystery as well, the case begins to unfold in unexpected ways...