With the gang trapped in detention and out of the way, Miss Quill accepts Dorothea's astonishing offer: to help remove the Arn from her head, and to reclaim her freedom.

Alongside Ballon, an imprisoned shape-shifter, Miss Quill travels in a metaphysical engine and is taken to impossible, extraordinary and dangerous worlds, collecting materials in preparation for her life-changing procedure.

After an emotional encounter with her surgeon and a nearly-fatal operation, Miss Quill must find a way to return to Earth: transformed and ready for war.