In this world, there are Strays - people who arrived from another world's Japan. These Strays were responsible for great disasters in the past so upon being spotted, they are meant to be killed by Executioners.
Menou, an Executioner, meets a Stray named Akari, who she kills without hesitation. Yet, Akari comes back to life. Searching for a way to kill her, this journey of Akari and Menou begins.