An 18 year old girl runs around the track and fractures her legs after hearing a bell ring for her final lap. A groom strangles himself to death at his wedding after hearing the clang of a spoon against a wineglass. A 73 year old man jumps out of the window on his wife's 70th birthday after hearing the bell chime from a toy train. Police sent to investigate these, nearly simultaneous, suicides discover one thing in common: the victims all mentioned a "Green Monkey" right before their death.

Meanwhile, a young girl named Yuka Irie locates Noriyuki Jissoji, a famous hypnotist from television, and tries to pay him to help her take care of the green monkeys that she says are following her. During their meeting, Jissoji discovers that whenever the girl hears a clapping noise she will revert to one of her multiple personalities, of particular interest to the entertainer is one where she proclaims to be a "friendly alien from Fratima's Seventh Nebula." The hypnotist ignores her plea for help, but instead decides to include the girl in his act.

In the mean time, Toshiya Saga, a specialist in multiple personalities, tells Sakurai Takanori, the investigator assigned to the suicide case, that he thinks hypnotism may be the weapon in the three people's deaths. Shortly after presenting this theory, it's discovered that white blood cells increased and that blood was dissolved unusually fast in the corpse of the groom, a reaction that is comparable to that of a snake bite, which is revealed to have been the man's phobia.

Before this could lead the investigation further, though, more suicides begin to pile up: a truck crushes a warehouse worker after hearing the clang of a pipe, a young girl locks herself in a freezer after hearing the chime of a clock, and a woman gags herself to death in a phone booth after hearing her key drop. Once again, all three utter the words Green Monkey before their death.

Saga explains that the hypnosis used might be to bring disturbing memories from the person's past and then simulate them happening again. Taking advantage of their disturbance as these events were likely intensified to the point of causing them to kill themselves to end the vision. While explaining this, the police see Yuka on TV, showcased in Jissoji's “Connection with the Universe” show, just as she starts to mention green monkeys. The police immediately investigate the studio, but find no useful information as Jissoji is unreasonable.

While this is going on, a seventh suicide takes place as a man uses his head to hit baseballs after hearing the ting of his bat. Following up their only lead, the police investigate where Yuka used to work, and discover that she was hospitalized a year ago for anorexia. They then travel to the nearby clinic and discover that the girl had stayed there for five weeks, and that past the first week she began to gorge herself on food. However, a nurse tells the police, in secret, that she walked by the girl's room at night after the gorging began and saw a social worker, nicknamed "Rat", practicing hypnosis on her. She then reveals that, after the five weeks, both Yuka and Rat left the hospital at the same time. This discovery is given urgency as another suicide takes place, this time hypnotist Jissoji is the victim, and the police desperately begin to seek out Rat, who they believe to be the mastermind behind the string of deaths.

Saga ventures alone and begins to follow Yuka around, at which point he is introduced to Yuka's third personality: Reiko, an individual who wears a patch on her left eye. She seduces Saga, and he ends up staying the night with her. In the morning, the young man awakes to find Yuka hiding from him. Saga apologizes for his actions, realizing that Reiko's intent was likely the opposite of Yuka's, and convinces the young girl to be brought to the police station for questioning. Once there, the authorities meet all of her multiple personalities, which include Yuka, Fratima, Reiko, and a fourth unknown persona. In the last personality, the girl leaps atop her chair and starts screaming, shattering the surrounding glass and causing everyone in the room to slip out of consciousness for nine minutes. With everyone passed out, the girl quickly flees from the building.

Following this incident, all of the authorities present for the questioning slowly begin to commit suicide as one washes his face with flames while another one rams a coat rake through his head. The horrible string of events allows Saga to finally connect that the start of each suicide is brought on when the beating of metal is heard. This revelation is discovered too late, though, as Sakurai commits suicide by shooting himself in the head after hearing the triangles played during a concert.

By coincidence, police investigate an apartment after a neighbor heard a scream emanate from it, and Saga finds the one called Rat inside, his eyes' pried open in front of a TV screen. The man, in a nearly dead state, begins to explain that his toying with Yuka had unleashed the multiple personalities, but he relates how a mysterious monster eventually showed up in the girl just before Rat dies.

Afterwards, Saga returns to the police station and meets Yuka hanging from the ceiling, as he finally discovers that her fourth form is, in fact, "Green Monkey," the monster Rat was talking about. The young girl then leaps from the ceiling as she tries to bang nearby metal objects to unleash Saga's hallucination. In the process, the girl eventually falls out of a window while trying to push Saga off the ledge. As she clings to the windowsill, the girl reverts back to Yuka, prompting Saga to attempt to save her. The young man makes a crucial mistake, though, as the girl, when nearly saved, utilizes the opportunity to bang a metal pole, triggering Saga's eventual demise. The investigator then drops Yuka, causing her to hang by the ledge again. The girl, in her Green Monkey visage, then explains to the hallucinating Saga that everyone has "Green Monkeys" inside of them, as her arm fractures and she plummets to her death.