Sixteen-year-old orphan Yuuto Amakawa has terrible cat allergies. So when he finds himself suddenly face to face with a beautiful girl who makes those allergies act up one morning, he's left wondering about her identity. And when she saves Yuuto and his friends from a demon spirit with a very real, very sharp sword, she reveals herself as a sworn protector of the Amakawa family and a demon spirit to boot! Poor Yuuto, now that he's living with an otherworldy samurai catgirl, his runny nose is the least of his worries!