While investigating the world of Parisian tramps, journalist René Savary, himself disguised as a poor beggar, notices the strange behavior of a little boy. He follows the brat after he has sneaked out of a barge, catches up with him and takes him to his home. There, the kid tells him about himself : his name is Jean, his mother has disappeared and since then he has been left to fend for himself. Moved by the lot of the wretched boy, Savary, with the aid of his friend Pierrot, undertakes to solve the mystery...