Olga Barkovskaya receives an invitation to participate in a family-friendly science show, where she must solve mathematical puzzles from the audience. During the live broadcast, a viewer calling himself Monti contacts the studio and proposes a game to Olga based on the Monty Hall paradox, with a human life at stake. Thinking it's a cruel joke, Olga refuses to play, and soon the police find a body in the woods with a note addressed to Olga. Convinced that Monti killed the person because she refused to play with him, Olga is willing to assist Kartashova, who is leading the investigation. But soon Monti contacts Olga again and offers to play once more. Now Olga must either figure out Monti's true identity or accept his challenge and try to prevent another murder.