Dani Fayel's backbreaking job as a stable hand at The Freaks' Fair isn't glamorous, but for someone looking to keep to the shadows? It's perfect. Neither paranormal freaks nor human norms ever notice her. But when Dani is forced to either let a child die in order to maintain her anonymity or choose to save him and risk being discovered, she finds herself unwillingly thrust into the spotlight.

Cain, Hell's premier assassin and a predatory incubus, is close to fulfilling his final contract and earning his freedom. Hired by the Archangel of Death to find his daughter, Cain follows her trail to the Fair. He works undercover, certain his mark is inside one of the portable canopies or traveling cages. Dead or alive, she equals his ticket to freedom.

When Cain's objective is divulged, Dani realizes she can't outrun her destiny. As the fabled Key - the one soul with absolute free will who is tied to the three realms of Heaven, Earth and Hell - only she is capable of opening the gates for the Four Horsemen of the Apocalypse. Whoever frees the Horsemen allegedly commands them. Now both Heaven and Hell are vying for her allegiance, and there are no limits as to how dirty they'll fight to obtain it. With Hell raining indiscriminate chaos as Heaven maneuvers its players around her, Dani realizes her would-be assassin may be the only man capable of helping her survive.