At the center of the events of the Russian crime drama series "Cain's Mark" is the story of an unusual married couple. They met at a fairly advanced age. Elizaveta had already become a widow, and Valentin had divorced his ex-wife. Both of their children have grown up and live independently. Contrary to everyone's concerns, the stepbrother and stepsister found a common language. The new unit of society comfortably settled in a coastal town, intending to grow old together. But one day, Lisa and Valya decided to take in an adopted son...