When Mio sneaks the family's katana--a priceless ancestral sword--from her parents' attic, she just wants to spice up a costume. Awakening the power within the sword unleashes a terrible, ancient evil onto the streets of unsuspecting London. With creatures straight out of Japanese myths stalking her and her friends, Mio realizes that if she cannot keep the sword safe and learn to control its legendary powers, she will lose not only her own life--but the love of a lifetime.