Watch Vím, že jsi vrah... (1972) Official Trailer

Dana Martinová, a young woman of loose morals, dies at the hands of a mentally unstable young man, Pavel Zach. In a fit of jealousy, he could not restrain himself and killed the girl with a blow to the head. The police find the corpse thanks to Dana's friend Eva Simonova. And it's Eva who becomes Pavel's second victim. The young man, afraid of being found out, attacks the girl and drowns her in the bathtub. Lieutenant Zeman, a criminal investigator, is put in charge of the double murder as his first case, assisted by his more experienced colleague Suchánek. However, the investigation initially goes nowhere. While Pavel's father tries to have his mentally ill son committed to a psychiatric hospital, Pavel commits another murder out of fear. Neither he nor the investigators know that Pavel has been a mere tool in the hands of someone else for some time...