The story takes place in Naples on the eve of Christmas. Rocco is a small boy, the son of Peppino and Mariu. But his parents are expecting the imminent birth of his little brother, Franceschiello, which must be born on 25 December. Seeing his parents busy in anticipation of the arrival of the little brother, Rocco is persuaded that Franceschiello will steal the love of his parents. It is at this time that Satan sends to Earth three Devils, Astarotte, Farfaricchio and Scarapino, with mission to convince a human being to prevent the birth of Jesus. Three Devils use the jealousy of Rocco and persuade it that if it prevents the birth of the child Jesus, it will prevent the same time to Francheschiello. They then reveal him a magic formula, "Opopomoz", which allows him to go back in the past until the eve of the Nativity. Rocco began to prevent Mary and Joseph arrive up to the barn where must be born Jesus. He eventually understand that it has been manipulated, but a little late: it will ...