The Berlin chief inspectors Nina Rubin (Meret Becker) and Robert Karow (Mark Waschke) solve their first case. 13-year-old Jo (Emma Bading) wanders around town crying. Her older brother Ronny (Theo Trebs) lives in a closed institution for delinquent youth. The panicked call from his sister leaves Ronny no choice. Although he is about to be released, he breaks out. Robert Karow begins his first day on homicide at a bloody crime scene. There is no trace of the corpse. A horrible place in an empty vacation home where Karow meets his new colleague Nina Rubin. With her assistant Mark (Tim Kalkhof) and the young trainee Anna (Carolyn Genzkow)Rubin and Karow begin to investigate. Karow soon suspects that it is a case from the drug scene.
Nina is skeptical, not least because her colleague Karow has a questionable reputation. Then events take a turn for the worse: Ronny and Jo flee from their brutal pursuers, and a body is found in a rubbish dump. When the investigators find out that the siblings are hiding in an empty hotel on the grounds of Berlin Brandenburg Airport (BER), a race against time begins... "Tatort: Das Muli" is a production of EIKON Media GmbH (producer: Ernst Ludwig Ganzert) commissioned by Rundfunk Berlin-Brandenburg (rbb) for Das Erste