One day, Hitoyoshi Yokoya wakes up to a mysterious nameless girl dressed as a maid ringing his doorbell, asking to be his servant. With her former master referring her to the Yokoya household, she offers her expertise as an assassin. Intimidated by her background, Hitoyoshi tries to send her away; but when she leaves something behind, he rushes to return it to her and is nearly hit by a truck in the process. With her...