"Sir Thursday" is the fourth book in the "Keys to the Kingdom" series by Garth Nix. It continues the story of Arthur Penhaligon, a young boy who must navigate a fantastical world ruled by the mysterious and powerful House. In this installment, Arthur is confronted with new challenges as he attempts to reclaim his rightful place as the heir to the House and its magical realms.
The novel explores themes of responsibility and identity as Arthur grapples with the conflicting obligations of his life in the real world and his duties within the House. As he meets various intriguing characters, including Sir Thursday, the titular character, he faces moral dilemmas that test his courage and resolve.