One morning, Hitoyoshi Yokoya is startled by the ring of his doorbell, where a silent girl in a maid uniform stands, requesting to become his servant. Sent by her previous employer to join the Yokoya household, she declares her specialization as an assassin. Unsettled by her deadly background, Hitoyoshi attempts to dismiss her; however, after she departs and forgets something, he chases after her to return it, only to almost get hit by a truck. The maid's quick reflexes save him just in time, convincing Hitoyoshi to let her stay.
Ironically, the girl possesses no abilities beyond the lethal arts she honed from a young age. Even so, Hitoyoshi welcomes her warmly, urging her to abandon her bleak, colorless past and embrace an ordinary life. To seal their bond, he gives her the name Yuki, setting her on a journey toward redemption and happiness.