When Kirari was young, her potential as an assassin was recognized by the killer Kuro Adaba, who took her under his wing. One day, Adaba, seeking to wash his hands of the assassin life, opened a Coffee Shop called "Yoi No Ashiato" (Footprints of the Evening), a specialty store for the underworld business. However, one year later, he was killed by someone searching for the "Key" that leads to the Adaba clan's information vault. Kirari, entrusted with the shop, tries to protect both the store and the "Key" as the Master, but...