Happy-go-lucky Haru Ichinose is an upbeat, all-smiles kind of girl. At a prestigious all-girls boarding school, Haru is one of the most popular girls on campus. The attention paid to her, however, is about to turn deadly. Enter Tokaku Azuma, a cold-blooded female assassin, who moves from one mission to the next with deadly precision. The heartless Tokaku has set her sights on Haru as her next target.

What should have been a routine hit turns into a far more complicated affair as Tokaku is unable to pull the trigger on Haru. As Tokaku copes with her strange, budding feelings, will she decide to save Haru's life or end it?