In the darkest depths of London, two brothers dwell in a tavern inhabited by dead, eldritch monsters. Axel Linden plays deadly card games for two simple reasons: to protect his brother Joey, and to obtain a bloody elixir that grants him access to every mortal emotion imaginable.

But when the elder Linden brother invokes the wrath of one of the ancient ghosts by stealing its blood, Joey and Axel have no choice but to flee the city they call home. The brothers escape with the Butterfly Circus, a mysterious group of performers that Joey is instantly drawn to; in particular, the alluring trapeze artist Dasha and the charming ringmaster James. He and his brother must keep Axel's crime a secret, but he senses that the circus has magical secrets of its own... secrets connected to the monster that pursues them. As they travel across post-war Europe the threat of the vengeful creature on their heels grows closer, and Joey must choose: the circus, or his brother?