A woman’s dead body is found wrapped in a khaki-coloured sheet and hung from the eaves of the upper floor of a run-down apartment building. A message seeming to claim responsibility for the crime has been pasted on the inside of the sheet, “Today, I caught a frog and experimented with a bagworm. As a result, it is extensively reported as a bizarre case by the media. Rookie detective Kotegawa Kazuya and the veteran Watase from the Saitama Prefectural Police’s First Investigation Division are put in charge of the investigation. Omaezaki Munetaka, who is an authority on criminal psychiatry, points out the childishness of the case and implies that it is likely to lead to serial killings. Kotegawa and Watase pursue the killer, but a second murder takes place before they are able to find any leads. This time, a crushed dead body is discovered in the trunk of a car at a factory for scrap cars. A similar message is found at the scene, and the criminal called Frog Man plunges the residents into fear and confusion. Will Kotegawa and Watase be able to reveal the identity of the Frog Man and arrest him?