A young Moscow professor Marina comes to rest in a sanatorium on the Volga. She dreams of an adventure, maybe a detective, at worst, a romantic one. And he gets everything in the first laziness in one bottle. The wind carried her hat to the pond, and, pulling it out, Marina saw a drowned man in the water. The police considered it an accident. But Marina is sure - this is murder. She noticed one strange detail ... But who to share with? She tells her secret to Fyodor Tuchkov, whom at first she considered a cretin, and the very next day he became her partner. A holiday romance is brewing, which she resists with all her professorial strength. But then another vacationer dies, who knew something about the drowned man. Marina has no choice but to trust Tuchkov again, especially since it turned out that he is a professional ...