While the Christian king of Portugal, Dom Sebastian is invading the Muslim Moors, the Grand Inquisitor takes advantage of his absence to seize power. Dom Sebastien rescues Zayda from the Inquisition with the intention to return her to her father, but the father wants her to marry the Moor leader Abayaldos against her will. Sebastien, in cognito, fights Abayaldos for Zayda, but is beaten and lays wounded. Zayda pledges to marry Abayaldos without a fuss in return for his sparing Sebastien's life.Back in Lisbon Sebastien's uncle, Dom Antonio, had usurped the throne, and when Sebastien identifies himself, both he and Zayda are thrown in prison. In his attempt to spare Zayda's life, Sebastien signs the instrument of abdication, but both are killed in their attempt to escape.