A German city is thrown into panic by a series of child murders, prompting both the police and the local criminal underworld to hunt for the killer. A compulsive whistler is eventually identified as the prime suspect and finds himself cornered by the city's organised crime network, who put him on trial in a kangaroo court before the authorities close in. The film is a landmark of early sound cinema, exploring themes of mob justice, psychological torment, and the nature of guilt.