Events are spiraling out of control inside Dragonwall, as Shayla's enemies seek to turn the tables on her, and Kale finds his position as the knights' commander challenged. When the queen asks Kale to report to a nearby village that has been raided, he jumps at the chance to get away from the politics and intrigue. Little does he realize he's about to uncover a terrifying secret that may change the course of history...

Meanwhile, the Iron Horse makes a surprise stop when Socrates and his team discover a train wreck filled with the mummified remains of an ancient crew. While studying the site, they make another unexpected discovery: The crater beyond Dragonwall is home to a previously unknown civilization; a race of humans living in a city of pyramids and possessing strange, powerful new technologies.

The crew's excitement is soon tempered by the realization that these timid farmers are not just shy of strangers... they are in fact slaves. Worse yet, their overseers are a race of creatures unlike any the crew have previously encountered. They are dangerous. They are powerful. And they are legion.

Clockwork Legion is Book 4 of Sedgwick's "Iron Horse" series, a follow-up to the best-selling steampunk series "The Tinkerer's Daughter."