Titus - a son and heir for the 76th Earl of Groan - is born. In the castle's kitchens, a new kitchen boy, Steerpike, rebels against the cook, Swelter, and runs away. He invades the bedroom of Lord Groan's daughter, Lady Fuchsia, who finds herself strangely attracted to him and takes him on a visit to the family's physician, Dr Prunesquallor. Steerpike accepts a place as Prunesquallor's assistant, meaning a dramatic rise in his status, and is taken into the confidence of the Earl's twin sisters, Lady Cora and Lady Clarice. Working on the sisters' feelings of injustice, he persuades them to set fire to the castle library, dear to their brother's heart. Steerpike knows - but the sisters do not - that the rest of the family is in the library at the time, with the doors barred.

Steerpike rescues the Groan family from the burning library and is the hero of the hour. Sepulchrave Groan goes mad with grief over the loss of his library. Believing he is an owl, he releases the owls in the Tower, and they attack and kill him. Titus becomes Earl of Groan. Flay annoys Lady Groan and is ordered out of the castle. He observes from afar the new Earl's coronation, at which Steerpike cuts a prominent figure.

Steerpike becomes the deputy of Barquentine, master of the castle's rituals. He imprisons Clarice and Cora, faking their deaths, kills Nannie Slagg, and continues to pay court to Fuchsia. Titus begins his education at the castle's dusty school, and Prunesquallor's unmarried sister, Irma, falls in love with Professor Bellgrove, now the school's headmaster. As the years go by, Titus finds the castle's rituals more and more hateful, focussing his hostility on Steerpike. He finds a forgotten passage, leading outside the castle walls. Venturing into the world outside for the first time, he meets the exiled Flay.

The elderly Barquentine clings onto his office. Frustrated, Steerpike kills him by fire, but he is left with his own face badly burnt. However, he takes over running the castle and continues to woo Fuchsia. Flay returns from exile and joins forces with Titus against Steerpike. They find him with the skeletons of Clarice and Cora, and Steerpike kills Flay and escapes. As Titus hunts Steerpike, Fuchsia refuses to hide him, killing herself instead. Titus and Steerpike fight to the death.