The two-part television film "Northern Lights: What the Mermaids Keep Silent About" from the detective series about writer and amateur sleuth Agatha Sever once again immerses viewers in the atmosphere of the harsh romance of Lake Ladoga, where Agatha has settled with her literary agent and faithful companion Marusya. As before, the plots of the detective stories Agatha writes do not compare to the dangers and intrigues the woman encounters in real life. This time, danger arrives with the criminal Semyon, who returns to Karelia after serving a long prison sentence. Thirsting for revenge, he kidnaps Marusya and demands payment for old debts. Agatha realizes that this concerns a terrible incident on the island twenty years ago, as a result of which a girl named Veronika was torn to pieces. The local doctor Pavel, whom Agatha has become friends with, tells the writer about the circumstances of the old case. Sever begins comparing the information she has gathered and increasingly comes to the conclusion that Semyon most likely served his sentence undeservedly, and if he was not the one who killed Veronika, then the prime suspect is none other than Pavel.
