First part:

The story is set on a desert island where Costanza and her younger sister Silvia have lived for thirteen years. Victims of a shipwreck, the two women believe they have been abandoned by Costanza's husband, Gernando. Grief-stricken and resentful, Costanza carves an inscription on a rock demanding revenge for this supposed betrayal. Meanwhile, Silvia returns full of enthusiasm: she has found a tamed deer that had wandered off. Her sister's spontaneous joy reminds Costanza of her own misfortune and awakens memories of a world Silvia has never known.

Left alone, Silvia sees a boat in the distance. Frightened at first, she thinks she sees a sea monster, but soon she makes out human figures on the beach. It is Gernando, accompanied by his friend Enrico, who has come to find Costanza and Silvia. Gernando has finally regained his freedom after thirteen years as a pirate's captive. Intrigued but wary, Silvia goes into hiding to watch the two men. Gernando sets out to find the survivors, while Enrico expresses his gratitude to his friend who once gave him his freedom. Silvia, troubled, wonders about the new feelings this encounter has awakened in her.

Second part:

While exploring the island, Gernando discovers the inscription left by Costanza and fears that she is dead. In desperation, he begs Enrico to abandon him on the spot and tell his father of the tragedy. But Enrico refuses to let him sink and orders the sailors to restrain his friend and force him back on board. A short time later, Silvia meets Enrico. She is disturbed by this man, who embodies everything Costanza has always warned her about. Despite her initial distrust, Enrico manages to reassure her and reveals that Gernando never left Costanza: he was kidnapped by pirates while the two sisters were resting after the shipwreck.

Silvia and Enrico's sincere exchange gives way to a burgeoning emotion between them. They promise to meet again when Costanza and Gernando are reunited. Returning to the rock, Costanza finally meets Gernando. Upset, she faints. When he goes to find water to revive her, Gernando is carried away by the sailors, as Enrico has ordered.

Enrico arrives shortly afterwards, helps Costanza regain consciousness and explains the situation to her. In a moving finale, Gernando manages to free himself and is reunited with Costanza. Misunderstandings are cleared up and the couple reconcile in a touching embrace. Enrico, moved by his own impulse of love, asks for Silvia's hand in marriage. The two couples celebrate their newfound happiness, symbolising the triumph of love and loyalty in the face of adversity.