A gas leak in one of the entrances of a Moscow apartment building turns into a massive theft — after the liquidation of the accident, residents do not find money and valuables. Artyom and Mukhtar find a package with an ampoule of a substance in the trash, which is used as an additive to natural gas to detect it. In one of the institutes of organic chemistry, Lena Brusnikina finds out who had such an ampoule.

Evgenia Semenovna is hospitalized after the theft of a collection of cacti from her apartment, among which is her favorite, blooming once every twenty-five years. Granddaughter Svetlana submits a statement to the police. Suspicion falls on Evgenia Semyonovna's sister Claudia. This woman has been jealous for many years that Evgenia lives in a separate apartment with a loving husband. In addition, it turns out that Evgenia decided to give her apartment, previously bequeathed to Claudia, to her granddaughter Svetlana, and this is already a motive.