The famous chemist Elsa Stern comes to Russia to congratulate her half-brother Leonid on the anniversary. At a gala dinner, Leonid suddenly dies. The investigation establishes that he was poisoned, and poisoned by one of the guests. In addition to Elsa, three of Leonid’s friends came under suspicion: Zhilin, Ivanov and Fedotov, as well as their wives. Each of them had both the motive and the opportunity to commit this terrible crime. But employees of the private detective agency "Intellect" soon narrowed the scope of their search and turned their attention to a certain chemical laboratory that was producing reagents for Elsa Stern...