Goffredo is leading the Crusader army in its siege of Jerusalem. Goffredo is joined by his brother Eustazio, his daughter Almirena, and the knight Rinaldo. As Goffredo sings of the coming victory, Rinaldo declares his love for Almirena, and Goffredo confirms he will give them his blessing if Rinaldo is victorious in battle. Almirena urges Rinaldo to fight boldly and assure victory. As she departs, a herald announces the approach of Argante, a general of the opposing Saracen army. Argente requests a three-day truce to which Goffredo graciously agrees. After Goffredo leaves, Argante waits the powerful sorceress and Saracan Queen, Armida—who is also his lover. She appears and informs Argente that the Saracens’ only chance of victory lives in depriving their enemies of Rinaldo’s support. A task which she is prepared to undertake.
Rinaldo and Almirena affirm and celebrate their love. Suddenly, they are interrupted by Armida and her forces who abduct Almirena from Rinaldo’s embrace. Goffredo and Eustazio arrive to find Rinaldo in mourning. He tells the brothers what has happened and they propose the trio visit a magician named Magus who may have the power to rescue Almirena.
On the seashore, Goffredo, Eustazio, and Rinaldo draw closer to the magician’s lair, when suddenly a beautiful woman appears on boat. She promises Rinaldo that she can take him to Almirena, and he impulsively jumps on board, much to his companions’ dismay. Rinaldo sets sail with the mysterious woman and vanishes.
Almirena is being held captive in Armida’s palace. She is gaurded by Argante who, overcome by her beauty, confesses that he has fallen in love with her. He promises to defy Armida’s commands and free Armida if she, in turn, promises to return his love. Meanwhile, Rinaldo is brought before Armida. He angrily demands that Almirena be set free. Armida, drawn to Rinaldo’s noble spirit, finds herself falling for the daring knight. She attempts to seduce him by using her powers to assume Almirena’s form. Rinaldo suspects trickery and departs. As Armida grapples with Rinaldo’s rejection, Argante appears. Armida, still in Almirena’s form, tricks Argante into repeating his earlier promise of love and freedom. The sorceress swiftly resumes her own form and denounces his infidelity, vowing vengeance as she departs in fury.
Goffredo and Eustazio have found the magician Magus and are told that Almirena is being held captive in Armida’s. Magus bestows magic powers upon the brothers so they can enter Armida’s palace unharmed.
Armida is preparing to kill Almirena. Rinaldo, now in captivity as well, is powerless to stop her. Suddenly, Goffredo and Eustazio arrive and aid the couples’ escape. The four friends celebrate their reunion.
Argante and Armida are reconciled by eminent danger from their common enemy. They prepare their troops while Goffredo’s army advances, led by Rinaldo. A battle commences and when the dust clears, Rinaldo and Goffredo emerge victorious. Rinaldo and Almirena celebrate their love as Goffredo forgives the beaten foes and sets them free. All join in a chorus of reconciliation.