This novel was first published in 1764. The plot takes place in Italy in the Twelfth century. The main characters of the book include: Manfred, the illegitimate Prince of Otranto; Hippolita, Manfred's wife; Matilda, 18, Manfred's daughter; Conrad, 15, Manfred's son - "the darling of his father"; Isabella, Conrad's fiance; Father Jerome, a priest; Theodore, a young peasant and the actual Prince of Otranto; and the Knight of the Gigantic Saber, Isabella's father.
Manfred, the illegal prince of Otranto, arranged a marriage between his young son, Conrad, and the daughter of the Marquis of Vicanza, Isabella. The people of the town attributed the marriage of Conrad at such a young age to Manfred's fear of an ancient prophecy. The prophecy stated that the Castle and Lordship of Otranto should pass from the present family whenever the real owner should be grown too large to inhabit it.
On the day of the wedding, a large helmet appeared in the courtyard of the castle. When the family and all the guests saw the helmet, they noticed Conrad, crushed, underneath it. Theodore noticed that the helmet looked like the one on the statue of Prince Alfonso the Good. Another peasant rushed to the chapel to compare the two helmets. When he returned he told the crowd that the helmet was missing from Prince Alfonso's head. Manfred accused Theodore of being a magician and murdering the heir to Otranto.
When evening came Manfred told Isabella of his plans to divorce Hippolita and marry her in hopes that he might have a male heir. Having been told all this, Isabella ran away through the underground tunnels of the castle. In no time she was lost, but she found Theodore in the tunnels and he helped her find her way out to a nearby church. Manfred searched for the missing Isabella and instead found Theodore in the underground passage. He threatened Theodore for assisting Isabella. As this was happening, Manfred's servants told him that there was a giant sleeping in the hall. Manfred went out to investigate, but the giant had already left.
Father Jerome came by the next day to tell Manfred and Hippolita that Isabella had taken sanctuary in the church. Manfred took Jerome aside and asked him if he would help him divorce his wife and marry Isabella. Father Jerome avoided this by informing Manfred that Isabella might be in love with Theodore.
Manfred confronted his prisoner who said he helped Isabella escape but he had never seen her. Manfred called for Theodore's immediate execution and Father Jerome was brought in to give absolution to Theodore. In preparation for his execution, Theodore's collar was loosened revealing a birthmark. Theodore's birthmark meant that he was Father Jerome's son. Theodore was born before Jerome entered the church. Manfred offered to stay the execution on the condition that Father Jerome would hand Isabella over to him.
The Knight of the Gigantic Saber, the rightful heir of Otranto, arrived at the castle gates. He demanded that Isabella be released and that Manfred give up the throne. If Manfred did not, then the Knight wanted the satisfaction of a fight to the death. Manfred invited the knight into the castle. He hoped to get permission from the Knight of the Gigantic Saber to marry Isabella and keep his throne at Otranto. The knight entered the castle with five hundred armed men and one hundred more carrying a...