Stefan Lanz, a successful young stockbroker, is a member of a yuppie clique that meets regularly at a castle near Vienna for group sex weekends. When Lanz is taken for the second time in the hospital shortly before a harmless knee operation, he becomes suspicious. He gains access to his medical record and discovers to his horror that the word "positive" is entered under the heading HIV. Lanz concludes that someone from his clique has infected him. He decides to take revenge on each one and escapes from the hospital. Shortly thereafter, Commissioner Moser has to deal with two murders that seem to have nothing to do with each other. But then it becomes apparent that they were committed with the same murder weapon. He finds out that the victims must have known each other, because both knew a man: Stefan Lanz...