The famous mathematician Lungin was murdered in his hotel room, and an expensive diamond was stolen from his safe. The day before, a tipsy Lungin let slip to the students of mathematics professor Olga Barkovskaya that the safe code opens with ten digits associated with Pythagoras. Senior investigator Svetlana Kartashova has four students under suspicion, and each of them is pointed out by one or another evidence. But Olga, after analyzing the numbers of the code, finds a thread that will lead her to the real killer.