Manfred, the prince of Otranto insists that his only sickly – son of 15 years must marry the daughter of Vicenza, Isabella. His wife, Hippolita vehemently opposes this-but Manfred simply accuses her of sterility and incapable of begetting more sons. Afraid of the fulfillment of an ancient prophecy- “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”.
The said wedding is to be hold on Conrad’s birthday. Meanwhile, he is 15 years and on a fateful day, Conrad is nowhere to be found. A servant who is sent in search of Conrad returns, breathless and frantic and reports that a gigantic helmet has crushed Conrad to death. Manfred and his servants try effortlessly to bring out the mangled body of Conrad under the giant helmet. Hippolita, Manfred’s wife and Matilda their 18-year-old daughter return to the castle to mourn the death of Conrad. On her part, Isabella, yet to be married to Conrad joins Hippolita and Matilda, to mourn the gruesome death of Conrad even thoU9h she does not love him.
Meanwhile, Theodore, a young peasant reminds Manfred of the resemblance of the helmet and the black marble of Alfonso the Good. This infuriates Manfred who tries to punish the peasant but there is a sudden twist of fate as spectators rush in to tell Manfred that the helmet on the statue is missing. This makes Manfred to accuse the peasant of necromancy. Hence, he orders for his imprisonment without food. Hippolita, worried about the state of her husband sends Matilda to her father but the young lady is dismayed by her father’s uncouth and uncomplimentary remarks against her. Manfred tells her to go away because he does not need a daughter. She goes back to her mother and tells her that Manfred is well while concealing her inner thoughts. A servant is later sent to invite Isabella by Manfred. In obedience, Isabella goes to answer Manfred. He begins by telling her the importance of keeping up his line and blaming Hippolita for her “unfruitfulness. Hence, he has decided to divorce her – Hippolita and then to offer himself as Isabella’s new husband, since Curried is dead. Terrified by this unusual request, Isabella runs away and as Manfred attempts to pursue her suddenly, the “plumes of the fatal helmet” appeared at the window. After that, the portraits of his grandfather utter a deep sigh that distracts Manfred for a while, thus enabling Isabella to escape through a vault leading to the church.
Matilda waits anxiously for the return of Bianca, her servant whom she sends to find the whereabouts of Isabella. The two women discuss Matilda’s attitude. While Matilda wants to go to the convent, Bianca wants her (Matilda) to marry. Matilda admits being fond of Alfonso the Good’s picture and thinks that they have a common destiny. The two women discuss other secrets, especially that of Hippolita.
The noise from the chamber distracts the two women. Bianca thinks that it must be a ghost as she feels that the place is “haunted”. Through her window, Matilda finds out that it is Pie younq peasant whom she begins to talk to. But instead, the young peasant asks about Isabella, thus raising Matilda’s suspicion that Isabella must be in love with the young peasant. She is indeed disappointed to note that Isabella has never said anything about Pie peasant in spite of telling her every other thing. Then a servant suddenly comes to announce that Isabella has been found in the convent of St. Nicholas.
Presently, Manfred is in the apartment of Hippolita trying to ascertain the whereabouts of Isabella. Also, Father Jerome visits Manfred and Hippolita to talk about Isabella. Father Jerome specifically wants to find out whether Hippolita knows the cause of Isabella’s retirement to the convent. Manfred interrupts as he does not want Hippalita to know his motive. However, Father Jerome begins to give detail of Isabella’s story and once again, Manfred interrupts and Hippolita, suspecting that he does not want her to know his intention, leaves the two men to continue the discussion.
Meanwhile, Manfred makes concerted effort to convince Father Jerome of the need to have a successor (heir). He claims that he is even related to Hippolita which makes their marriage, unlawful. This sudden revelation startles Father Jerome who procrastinates At this juncture, Manfred tries to find out if there is a relationship been the young peasant arid Isabella. Father Jerome, sensing that this is an opportunity to save the grave situation, at firms that Isabella is the young peasant’s lover arid H jealousy angered by this news, Mantled summons the young peasant and interrogates him about his connection with Isabella. At this point Matilda and Bianca who are on their way to Hippolita’s apartment overhear the conversation of the men. They also real, that the young peasant whose name is Theodore has a striking resemblance with the picture of Alfonso the Good. Manfred, in his judgment orders that the young peasant he beheaded. On hearing this, Matilda faints and Bianca screams that the princess is dead. While Matilda is carried away, Manfred orders Theodore to kneel down to receive hrs punishment. Meanwhile, Theodore asks to be given the opportunity to confess his sing and his request is granted. Manfred thus invites Father Jerome, hoping that through him, he (Manfred) may get to know more about the boy. Father Jerome desperately appeals for leniency for the young peasant but Manfred insists on carrying out the punishment (to behead him). As bather Jerome goes to pray for the boy, his shirt falls down and Father Jerome beholds the “mark of a bloody arrow’ on Pie boy thus confirming Theodore as his biological son. Father Jerome now tells his own story, being the count of Falconara (Sicily). He begs Manfred to spare Theodore’s life but Mantled gives a condition that he can only do so in return for Isabella, but both Father Jerome and Theodore object vehemently in other to save Isabella. Before Manfred reacts to their objection, the trampling of horses and the sound of trumpets are heard. Ag, the plumes on the weird (enchanted) helmet start to shake and nod as if they are bowing.