Irren ist männlich.

Lawyer Thomas Neumann is extremely successful in his career and happily married to his wife Bettina. Nevertheless, he has a mistress on the side, who one day reveals to him that she wants him to have a child. Thomas agrees, but a sperm test reveals that he is infertile. Now he asks himself the question: who is the real father of his children? His brother Johannes, a priest, helps him to find out the truth. Thomas suspects two former friends with whom he and Bettina used to go on vacation. He invites them to his home so that he can carry out comparative studies in peace, as genetic testing didn't exist back then. One friend has gone mad, the other is blind. The two of them cause Thomas' household a great deal of confusion, especially as he has to entertain an important business partner from overseas at his home.