Mathilde, 13, is frustrated by the state the world is in today – with all the pollution, poverty and inequality. She decides to form a detective agency with the aim to help the weakest members of society. Unfortunately, the only two candidates interested in joining her agency are Tobias and Gustav. With lots of persistence and against all odds, this motley crew succeeds in thwarting the plans of a notorious mobster, but the detectives also catch the attention of an ambitious police superintendent, who believes that a child gang is ravaging her city.