In the psychiatric clinic at the "Steinhof", a spacious Art Nouveau complex, mysterious deaths occur among the patients. The clinic pathologist confidently turns to his colleague Dr. Graf and asks him for help. All patients died in the same way: they drowned in a bathtub. Graf informs Moser, who wants to take up the investigation. But the prison management refuses a police investigation. Moser decides to be admitted to the institution as an "alcoholic". He entrusts Rex to his assistant Stockinger. But before he knows it, Rex flees and hides on the hospital grounds. As a result, he gets Moser into big trouble, because where should he hide Rex? Meanwhile, the situation escalates dramatically, Prohaska, the ward nurse, wants to eliminate Moser. With a daring jump through a closed window, Rex saves his master's life...