Monsters. They rush at the victim and instantly tear it to pieces. Their teeth sink into the heels of careless swimmers, they are able to bite the most powerful fishing rod in half... Piranhas are the name of these creatures. Where did the South American fish come from in the cold Russian river? The great detective Felix Kuropyatkin simply has to solve this riddle: after all, the swimming competition is about to begin. Calling on his faithful friend for help, he begins an investigation...