Brothers Dima and Alyoshka became very friends with the new biology teacher. Vadim Ivanovich Korenkov was almost a professor, and therefore a very absent-minded person. Alyoshka even had to take charge of him so that Korenkov would not wear different shoes and tie his tie inside out. But then one day Vadim Ivanovich had a really serious problem. Black Margot, a unique artificial pearl, the result of a scientific discovery, has disappeared. Was she stolen? Or did the forgetful scientist himself put the pearl somewhere unknown? Alyoshka and Dima were determined to find what was missing...