The story begins on the verge of a wedding. The husband to be is killed in a horrific freak accident – he is crushed to death by a gigantic helmet that falls on him from high. The death is part of a prophecy (another recurrence in Gothic novels) that warns 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 prophecy, and the fear it strikes into the heart of the Manfred, the father of the husband-to-be, leads to a series of events that end in tragedy for the man.
Otranto was lauded in its own day, until Walpole revealed that it was satirical fiction. After that seemingly tiny admission, the people turned against it. Even then, however, its importance could hardly be contained. The Castle of Otranto has continued to influence in the 250+ years since it was first published and is considered a classic.
If you want a physical copy of the book, the Oxford World’s Classic edition is a good starting place.