It concerns the story of the prince Manfred and his desire to avoid a curse placed upon his ancestors, which places him at risk of losing his castle and title. Caught in the middle of the prince's desperate bid to secure his position are Theodore, a young man wrongly imprisoned under false accusations by the prince, and Isabella, an innocent maiden who Manfred wants to marry after his son met a terrible fate.
Walpole's Otranto arguably sets the stage for the creation of the Gothic horror genre, which would inspire later authors down the line.
His work also introduces the tropes that would be prominent within the genre, and still be recognizable today.
The Castle of Otranto provides examples of:
- Abdicate the Throne: A variation. Faced with the loss of both his children and the true heir of Otranto revealed, Manfred complies to step down from his title alongside Hippolita and retire into the church.
- And Now You Must Marry Me: Manfred tries to do this to Isabella in order to try to avert the destruction of his line, setting up the conflict.
- Arranged Marriage: Isabella was engaged to marry Manfred’s son at the start of the novel.
- Crazy Jealous Guy: Manfred, starting an enduring Gothic villain tradition.
- Curse: When a giant helmet falls on and kills Manfred's son, Manfred fears it is the result of a prophecy laid down by his ancestors that states "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."
- Dead Person Conversation: Frederic is visited by an apparition, who commands him to not go through with marrying Matilda.
- Distressed Dude: Theodore is imprisoned and rescued by Matilda.
- Domestic Abuse: Not just due to Values Dissonance, either, because Isabella is initially terrified of Manfred because of how she's seen the way he treats his wife and daughter.
- Hoist by His Own Petard: Manfred ultimately brought ruin to himself when he unknowingly stabbed his own daughter in the insane belief Isabella was meeting with Theodore.
- Horrible Judge of Character: It's a wonder why Isabella's father thinks Manfred is a noble man, not knowing what problems he's caused his daughter before he got to Otranto.
- Homage: To Walpole's idol William Shakespeare, which the author was always up-front about.
- It's Probably Nothing: Bianca and Matilda agree the strange noises must just be "the wind."
- I Want My Beloved to Be Happy: Matilda is willing to give up Theodore to Isabella.
- Love Triangle: Isabella loves Theodore, who loves Matilda, who loves him back.
- Luke, I Am Your Father: Father Jerome is Theodore's father.
- My God, What Have I Done?: When Manfred realizes he fatally attacked his own daughter, he is immediately horrified at his wrongdoing.
- Nice Job Breaking It, Hero: Nice job mortally wounding the knight who came looking for Isabella, Theodore...
- Offing the Offspring: Manfred, in full-on If I Can't Have You… mode, busts in on what he thinks is a tryst between Isabella and Theodore, but instead of Isabella, it is Matilda who Theodore is meeting with, and she is tragically stabbed to death.
- Really Royalty Reveal: It's later revealed that Theodore is the true heir to Otranto.
- Rescue Romance: Isabella falls in love with Theodore after he helps her escape Otranto, while Theodore falls in love with Matilda when she rescues him from imprisonment.
- Supernatural Aid: The ghosts of the castle for Isabella.
- Taking the Veil: Hippolita retires to join the church along with her husband.
- Trope Codifier: For Gothic tropes.