When Mio steals the family's katana – a priceless ancestral sword – from her parents’ attic, she just wants to spice up a fancy-dress costume. But the katana is much more than some dusty antique and her actions unleash a terrible, ancient evil onto the streets of unsuspecting London. Soon Shinobu, a fearless warrior boy, appears to protect Mio – and threatens to steal her heart. With the gods and monsters of Japanese myth 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.