Theodore, having been locked in a tower by Manfred, is freed by Manfred's daughter Matilda. He races to the underground church and finds Isabella. He hides her in a cave and blocks it to protect her from Manfred and ends up fighting one of the mysterious knights. Theodore badly wounds the knight, who turns out to be Isabella's father, Frederic. With that, they all go up to the castle to work things out. Frederic falls in love with Matilda and he and Manfred begin to make a deal about marrying each other's daughters. Manfred, suspecting that Isabella is meeting Theodore in a tryst in the church, takes a knife into the church, where Matilda is meeting Theodore. Thinking his own daughter is Isabella, he stabs her. Theodore is then revealed to be the true prince of Otranto and Matilda dies, leaving Manfred to repent. Theodore becomes king and eventually marries Isabella because she is the only one who can understand his true sorrow.