London, 1923. Dragons soar through the skies and protests erupt on the streets, but Vivian Featherswallow is determined to follow the rules, secure an internship studying dragon languages, and ensure her younger sister never has to risk growing up Third Class. By midnight, however, Vivian has started a civil war. With her parents arrested and her sister missing, the safety Vivian worked for has collapsed. She accepts a mysterious job at Bletchley Park, where she is recruited as a codebreaker for the war effort. Her task is to discover the secrets of a hidden dragon language; if she succeeds, her family can return home, but if she fails, they will all perish. As Vivian delves into the assignment, she realizes the world she grew up in is not as safe as she believed. Amidst a clandestine, slow-burn romance and a web of secret identities, she must ultimately decide which war she is truly fighting.