When Mio sneaks the family’s katana — a priceless ancestral sword — from her parents’ attic, she just wants to spice up a costume. But the katana is much more than a dusty antique. Awakening the power within the sword unleashes a terrible, ancient evil onto the streets of unsuspecting London. But it also releases Shinobu, a fearless warrior-boy, from the depths of time. He helps to protect Mio — and steals her heart.