The plot of the novel centers on the conflict be-tween the forces of good and those of evil. Central to the novel is the fulfillment of an ancient prophecy that Otranto " should pass from the present family, when-ever the real owner should be grown too large to in-habit it", and Manfred attempt to prevent it. Manfred is aware of the ancient prophecy and is afraid it might be fulfilled because he is not the legitimate owner. This fear prompts him to arrange between his ailing son, Conrad and Isabella who belongs to the line of the true heirs. Since Frederic is presumed dead, it is easy for Manfred to gain approval for the insipid union. But this is not to be. Divine providence intervenes and Conrad is mysteriously killed just before the wedding. Clearly, Manfred symbolizes evil as he shows his intention to deprive the legitimate heir of the Castle of his inheritance.
Divine providence is the force which he has to face as supernatural events intervene to thwart his every move. Evil also manifest itself in Manfred's forced attempt of marry Isabella himself. The conflict has now intensified. As Isabella flees from him, divine providence in the form of the supernatural again intervenes. The portrait of Manfred's own grand: father hanging in the room heaves a "deep sigh" and leaves its frame. This serves as a distraction to Manfred, and gives Isabella time to escape. This incident issifignificant in two respects.
First, it demonstrates that even Manfred's own grandfather disapproves of his evil plan. Secondly, it provides Isabella valuable lime to make good her escape. The conflict now resolves itself into whether the evil Manfred can capture and force Isabella to marry him or lose her. The conflict is further complicated by the intervention of Theodore and Friar Jerome.
A further complexity to the conflict is brought about by the appearance of Frederic who is, at that moment, the rightful heir to Otranto. Manfred, the strategist, soon persuades Frederic to consent to a marriage between Manfred and Isabella and Frederic and Matilda. In one deft swoop, he attempts to unite the legitimate to prevent the fulfillment of the ancient prophecy. Divine providence again intervenes: this time, in the appearance of a mysterious figure which commands Frederic to "forget Matilda". This happens at the very moment Manfred is about to declare his intention to divorce Hippolita, Matilda's mother, which weakens Frederic's resolve to many Matilda.
Shortly after this, Manfred in a fit of jealousy inadvertently kills his own daughter, Matilda With this action, the bait dangled in front of federic is gone for good. The wicked Manfred in a bid to kill Theodore, whom he sees as a rival, deprives himself of what is effectively his ace —Matilda. In the end, therefore, the forces of evil are defeated.
Points to Note:
(1) Identification of the main conflict.
(2) Prevention of fulfillment of the ancient prophecy.
(3)Arranged marriage between Conrad and Isabella, ending in Conrad's death.
(4) Manfred's attempt to forcefully marry Isabella and the intervention of divine providence.
(5) The appearance of Frederic despite the belief that he is dead.
(6) The intended murder of Theodore which becomes the real fate of Matilda.