Businessman Sergei Andronov and his housekeeper were shot dead in their apartment. At the same time, Andronov's younger brother, 35—year-old Andrey, an autistic and brilliant mathematician, was in one of the rooms of the apartment and may have seen the killer. Olga Barkovskaya helps the investigation, with whom Andrey communicates through mathematical formulas. Andrei himself, the wife of the murdered Albin, a former friend Ilya Riman, and the husband of the murdered housekeeper Stukalov fall under suspicion.