Finn's childhood in the valley is idyllic, but across the plains lies a threat. Engn is an ever-growing steam-powered fortress, that needs a never-ending supply of workers. Generation after generation have been taken away, escorted into its depths by the mysterious and terrifying Ironclads, never to return. The Masters of Engn first take Finn's sister, then his best friend, Connor. He thinks he, at least, is safe - until the day the ironclads come to haul him away too. Yet all is not lost, Finn has a plan. In the peace of the valley he and Connor made a pact. A promise to join the mythical Wreckers and end Engn's tyranny. But now on his own, lost and thwarted in the vastness of Engn, Finn begins to have doubts. Is Connor really working to destroy Engn? Or has he become part of the machine?

The wheels of the great machine turn again… Three years after the destruction of Engn, Finn is awoken by a shattering earthquake. As the people of the valley flee the ruins, rumours circulate that, across the great grass plain, the machinery of Engn is working once more. That Engn is to blame. Finn, haunted by his experiences and the thought Connor desperately needed him to do something, tries to make sense of what is happening. Past history and current events collide. Uncovering buried secrets and confused memories, Finn sees he has to return to the wreckage of Engn, to ensure its destruction was total, and also to find answers to his questions. To him, the long-ago events of the Clockwork War are history, but he learns others are still fighting the ancient battles between the upstart Mechnical guilds and the twenty-four Temple guilds. For them, the machinery of Engn is at the heart of everything. Diane refuses to come with him, thinking he’s obsessed with a machine long destroyed, thinking he needs to put the past behind him. But then Engn’s line-of-sight timing signals start to broadcast again…