In Trastevere, in Vicolo del Moro, Sora Emma, a widow lives with her two children: Bruno, the eldest, is a carter; Gigi, the youngest, is an apprentice to a watchmaker, Sor Venanzio.
Gigi loves to have fun, neglects work by attending idle companies and, despite being engaged, runs after every girl.
When he meets Lulu, a singer, he becomes her lover and for her he commits a theft in the shop where he works.
Sor Venanzio notices the theft and Gigi, to compensate him, steals his mother's savings.
Bruno, immobilized his brother's right hand, forces him to hit himself and then goes to turn himself in.