1921. Three seemingly unrelated terrible events took place in a provincial Siberian town. But Chekist Kamchatov is convinced that there are links between the murders and, moreover, that in the last case, the suicide of a young security guard was, in his opinion, clearly staged...

A group of scientists is sent to the planet Arkanar to help the local civilization, which is in the Medieval phase of its own history, to find the right path to progress. Their task is a difficult one: they cannot interfere violently and in no case can they kill. The scientist Rumata tries to save the local intellectuals from their punishment and cannot avoid taking a position.