Two rustics, Gil and Menga, are looking for a lost donkey when they spy two men preparing to fight a duel. Lisardo, one of the men, is angry that anyone as low-born as Eusebio, the other, should aspire to marry Julia, Lisardo’s sister.

Eusebio explains by telling a miraculous story. He was one of two infants abandoned beneath a wayside cross. Taken home by a shepherd, the famished baby bit the breast of his foster mother, who threw the child into a well, where his rescuers found him floating safely with arms crossed. Later the house in which he was living burned, but the fire broke out on the Day of the Cross, and once more he survived unharmed. More recently, in a shipwreck, he floated to safety on a raft of two crossed planks. He explains that since he obviously acquired nobility by devotion to the cross, he deserves Julia. Lisardo denies the claim and they fight. As happened before in Eusebio’s life, no harm comes to him in a dangerous situation. As Lisardo lies dying of his wound, he begs in the name of the cross for Eusebio to save him. The amazed peasants report that they saw Eusebio pick up his dying enemy and carry him to a convent.

Back in Siena, Julia is fearful of her father’s discovery of letters she received from Eusebio. When her lover appears, wanting to take her away with him before she learns about her brother’s death, her father’s arrival forces him to hide and to listen to Curcio as he voices his long-held suspicions of his wife’s infidelity. Curcio is interrupted by the arrival of four peasants carrying the body of Lisardo. Julia, grieving, orders the killer out of her life forever.

Eusebio, brokenhearted, turns bandit and through his cruelty rises to command a troop of outlaws. Only captives mentioning the cross escape death at his hands. One day a bullet-creased prisoner is brought in carrying a volume titled Miracles of the Cross. He is Father Alberto, and in gratitude for having his life spared the priest promises Eusebio that he will be on hand to hear the bandit’s last confession.

News arrives that Lisardo’s father, having put Julia into a convent, is pursuing Eusebio with soldiers. Scorning danger, Eusebio lets his passion for Julia take him to the convent, where he finds her in bed. Before he can take her, he sees on her breast the same sign of the cross that is on his own skin. The mark tells him that she was the other child left beside the cross, his sister, and so he runs away. Julia, who tried to fight him off in her cell, begins to pursue him in masculine attire. She does not know why he refused to love her.

When the soldiers overtake him, Curcio wounds Eusebio fatally. Then the cross on the young man’s body reveals to Curcio that he slew his own son, abandoned with his twin sister because of the father’s baseless suspicions of his wife’s unfaithfulness.

With his dying breath, Eusebio calls for Father Alberto. Four shepherds arrive to bury his body. The priest also appears as he promised. He explains that because of God’s pleasure in Eusebio’s devotion to the cross, his soul was left in his body long enough for him to make his confession and to be redeemed.