Twelve years later, Griffin — now severely mentally handicapped — is cared for by his younger brother Ash, the leader of a gang of street kids in New York City.
One night, Ash encounters a mortally wounded man who gives him a vial of an unknown substance and an address in California; the man utters the words “banana fish” before dying.Ash begins to investigate “banana fish”, though he is impeded by Dino Golzine, a Corsican mob boss who had groomed Ash as a sex slave and heir to his criminal empire. In the course of his investigation, Ash acquires several allies: Eiji Okumura and Shunichi Ibe,
who have travelled from Japan to report on street gangs; Shorter Wong, a gang leader who controls Chinatown; and Max Glenreed, whom Ash encounters in prison while detained on a false murder charge. When Griffin is shot and killed in a fight with Golzine’s men, the group sets out to solve the mystery of “banana fish” together.Ash and his allies travel to the address in California,
finding a mansion occupied by a man revealed to be Yut-Lung Lee, the youngest son of China’s largest crime family. They later encounter the home’s true occupant, a doctor who informs them that “banana fish” is an untraceable drug that brainwashes its users.
Golzine intends to sell the drug to the United States government, which seeks to use it to overthrow communist governments in South America. The group is subsequently captured by Golzine’s men, who inject Shorter with banana fish and instruct him to kill Eiji. When Shorter begs Ash to kill him in a moment of lucidity, Ash fatally shoots him.
因变成废人的哥哥时常从口中说出“BANANA FISH”，亚修决定开始调查此事。格鲁兹在中美使用“BANANA FISH”发生政变，为了得手海洛因的走私路线进而与美国政治家和军人一同联手，并阻止亚修的调查。被冠上杀人的冤罪，亚修被关入大牢之中。在此认识了哥哥葛利夫于越南从军时的旧友马克斯・罗伯。马克斯自越战事件后，长年调查有关“BANANA FISH”的情报，两人便开始一同行动。
潜入位于加州的多森博士家中，亚修和马克斯查出“BANANA FISH”实际为含有复数生物碱的药物。并解明具有操控人的潜意识的功能。他们目睹了在格鲁兹的宅邸内，被施打了“BANANA FISH”的肖达因暗示的关系失去自我并想杀害英二，亚修遵从了肖达的愿望“杀了我，让我解脱”而射杀了他。