When Zeki Müller is released from prison after his sentence, he has only one thing on his mind: the loot from his last robbery. In order to get his hands on his money, which lies under the newly built gymnasium of the Goethe comprehensive school, Zeki applies for the advertised job as a caretaker. At the job interview there is a misunderstanding with the headmistress, Mrs Gerster, who hires him as a substitute teacher. Suddenly, instead of style and money, it's fist and Pythagoras for "Mr. Müller".

To make matters worse, he is assigned the problem class 10b, which has long since been abandoned by the teaching staff and labelled a loser class, and with which even the highly motivated Lisi Schnabelstedt is overtaxed beyond measure. With a harsh tone and unusual teaching methods, however, Zeki gradually wins the respect of his students, who feel understood and entrust him with their life's dreams.

Through Lisi and his students, Zeki begins to doubt his future: Is his fate as a criminal already sealed or does he deserve a second chance?