Chicago. Early morning. A man dies under the wheels of a bus. His face is disfigured beyond recognition; he has no documents on him. But a white box tied with black twine was found nearby, and in it was a human ear. Detective Sam Porter realizes that this is the sign of the serial killer U4O (“Killer of the Four Monkeys”), who has been operating in the city with impunity for ten years. After another crime, he leaves the inscription “Do not commit evil,” apparently connected with the ancient parable of the four wise monkeys. Porter's instinct tells him that the things of the deceased, and among them the diary of the killer himself, are a kind of mocking clues with which the maniac deliberately teases the detectives, challenging them... For the age category 16+