Hans-Jürgen Mannhardt, a retired head of the 12th Berlin homicide division and a part-time lecturer at the University of Applied Sciences for Administration and Legal Services, visits the Tegel prison with his students. There, he is approached by inmate Karsten Klütz, who has been sentenced to a long prison term for murder. Mannhardt recognizes him and vaguely remembers the case: Klütz was once a prominent second division football player. He was wrongfully convicted in 1998 and asks the former commissioner to review the files once more. Mannhardt and his grandson Orlando, who has just started his law studies, set out to uncover the truth. The case dates back ten years and the evidence was overwhelming, but then a crucial clue comes from none other than Theodor Fontane's novel "Unterm Birnbaum.".