Manfred, the Prince of Otranto, has arranged for his son Conrad to marry the beautiful Isabella, the ostensibly orphaned daughter of the Marquis of Vincenza. On his wedding day, Conrad is found in the court, mysteriously crushed to death by a giant helmet. Conrad’s death causes great concern for Manfred because an ancient prophecy has predicted that Otranto will pass to the rule of another family once the current ruler has outgrown it. The death of Conrad, who was Manfred’s only son and heir, means Manfred’s bloodline will end.

Conrad’s bloody remains cause a stir among the servants. A peasant identifies the giant helmet as belonging to a giant statue of Alfonso the Good, a former ruler of Otranto, located at the nearby Church of St. Nicholas. In response, Manfred accuses the peasant of treason and Conrad’s murder and orders him to be locked up within the helmet.

Manfred summons Isabella and declares that Conrad was not good enough for her. He plans to divorce Hippolita and marry Isabella. Isabella, frightened by Manfred’s sexual advances, runs away. Manfred chases Isabella through the castle until he is distracted by the moonlight reflecting off the giant helmet.