The Castle Of Otranto tells the story of Manfred, lord of the castle, and his family. The book begins on the wedding-day of his sickly son Conrad and princess Isabella. Shortly before the wedding, however, Conrad is crushed to death by a gigantic helmet that falls on him from above. This inexplicable event is particularly ominous in light 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". Manfred, terrified that Conrad's death signals the beginning of the end for his line, resolves to avert destruction by marrying Isabella himself while divorcing his current wife Hippolita, whom he feels has failed to bear him a proper heir.

The Castle of Otranto is a 1764 novel by Horace Walpole. It is generally regarded as the first gothic novel. In the second edition, Walpole applied the word 'Gothic' to the novel in the subtitle - "A Gothic Story". The novel merged medievalism and terror in a style that has endured ever since. The aesthetics of the book shaped modern-day gothic books, films, art, music and the goth subculture.
The novel initiated a literary genre which would become extremely popular in the later 18th and early 19th century, with authors such as Clara Reeve, Ann Radcliffe, William Thomas Beckford, Matthew Lewis, Mary Shelley, Bram Stoker, Edgar Allan Poe, Robert Louis Stevenson and George du Maurier.