When Arthur Clennam returns to England after many years abroad, his curiosity is piqued by the presence in his mother's house of a young seamstress, Amy Dorrit. The cold and forbidding Mrs Clennam isn't usually given to charity, so why has she made an exception in taking on Amy? Is she making reparations for a past wrongdoing? Does it have anything to do with Arthur's father's dying words and the pocket watch with its mysterious inscription, 'do not forget'?

Arthur befriends Amy Dorrit and involves himself in her family's affairs. If, as he suspects, his family have something to do with their ruin, the least he can do is pay off her brother's debt and give him the chance to start again. Arthur himself is planning to start again with a new job, and perhaps a renewed love for childhood sweetheart Flora Casby.

Flintwinch taunts Arthur's mother about the mystery at the heart of the House of Clennam, while Amy discovers that 'a gentleman' has paid off her brother's debts, and Arthur is invited to Twickenham to renew his acquaintance with the lovely Pet Meagles.

Amy receives a proposal of marriage which throws her into confusion, Arthur is taken on as a partner in Daniel Doyce's engineering works, and the French villain Rigaud arrives in London.

Amy accompanies her sister Fanny to meet Mrs Merdle, queen bee of London society. Meanwhile, Rigaud comes into possession of the Clennam papers, and Chivery the turnkey asks Arthur to put in a word for young John with Amy.