The murder case of businessman Petr Vinogradov, at first glance, seemed simple: the businessman was poisoned in his own mansion, no outsider could enter the house—so the killer must be “one of their own.” But at the moment of the murder there were too many “insiders”—twelve people. The investigation was entrusted to the best investigators in the capital, but they still couldn’t find the murderer. All the suspects loved and respected Vinogradov, and no one had a motive to kill him. In despair, the deceased man’s brother turns to help the famous criminal expert Dongro, and the fan of criminal puzzles, fueled by excitement, takes on this difficult case…