Astrid had always scoffed at her eccentric mother's stories about killer unicorns. But when one of the monsters attacks her boyfriend—thereby ruining any chance of him taking her to the prom—Astrid finds herself headed to Rome to train as a unicorn hunter at the ancient cloisters the hunters have used for centuries. However, at the cloisters all is not what it seems. Outside, the unicorns wait to attack. And within, Astrid faces other, unexpected threats: from the crumbling, bone-covered walls that vibrate with a terrible power to the hidden agendas of her fellow hunters to—perhaps most dangerously of all—her growing attraction to a handsome art student ... an attraction that could jeopardize everything.