A maid appears at the house of high school student Hitoyoshi Yokoya, asking to be hired; her background as a former assassin makes Hitoyoshi think twice.

Hitoyoshi is compelled to adopt a puppy that was left in front of his home; the maid has childhood trauma involving dogs.

Riko, Hitoyoshi's younger sister pays him a visit to see the puppy. Loving beautiful women, she easily got along with Yuki despite being an assassin. The maid’s skills awe Riko and decides to use Yuki as a subject for her independent research for school.

To experience a normal school life, Yuki enrols at the same school as Hitoyoshi; seeing boys rush off, she finds out the school store sells a rare yakisoba bread; Yuki heads out the window, leaving Hitoyoshi to worry whether she'll fit in or not.

After the Nitta incident, Yuki decides to become Hitoyoshi's bodyguard, sticking to him 24/7; when she is distracted for a moment, a woman named Grace appears and takes Hitoyoshi hostage, seeking to kill Yuki for prestige and revealing her dark past.

Finding enjoyment in teasing the former assassin, Grace returns as a gym teacher; after much harassment, she clues in Hitoyoshi of the changes occurring to Yuki.