In this world, there are Strays - people who arrived from another world's Japan. These Strays were responsible for a great disaster 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.