King Of Thieves centers on the charming, ten-year-old Barbu who lives with his poor family in a Ukrainian village. A promoter named Caruso regularly comes to town, offering to buy children from their parents, promising them a fantastic future in the West in his traveling circus. Caruso similarly lures Barbu to Berlin. But Barbu is not destined for the circus. Caruso turns him over to a gang of thieves who train children to steal. They punish the children if they don't steal enough or if they keep any of the spoils for themselves. Under such brainwashing, Barbu resolves to become the King of Thieves.
Meanwhile, Caruso buys Barbu's older sister, Mimma, and sells her to a Berlin brothel. When Barbu finds out, he dedicates himself to freeing his sister from a life of forced prostitution.