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 modern-day London. But it also releases Shinobu, a fearless warrior boy, from the depths of time. He helps to protect Mio and steals her heart.