A body mauled to death by dogs is found, and other heinous murders follow. Mysterious notes are left at the crime scenes, hinting the killings were done to punish each victim and the bodies are presented as if they are meant to be found. The killer only appears on rainy days and is witnessed wearing a frog mask. Police detective Sawamura (Shun Oguri) is on the case and to his surprise he finds out that the next target of the Frogman could be his beloved wife and child. Sawamura tries to corner the Frogman, using clues, but is devastated to find out that everything is a trap.