Valentine is sent away by Crome on a "secret mission", much to Katherine's dismay. Suspicious of her father, Katherine begins investigating events in London with the help of Apprentice Engineer Bevis Pod, whom she befriends after discovering he witnessed Tom chasing Hester. They discover that Valentine salvaged a monstrous ancient weapon called MEDUSA for London, and that the Guild of Engineers have reassembled it inside St Paul's Cathedral. The Cathedral's dome splits open to reveal MEDUSA, which is then used to destroy a much larger city pursuing London.