Suddenly, monsters known as Carriers emerged from tunnels that had connected to another realm. Humanity was attacked and further ravaged by an unknown virus, leading to countless deaths.

Saeki Kaito, a Purifier tasked with hunting down the Carriers, was frail and sickly—just like his younger sister. To earn money for her medical treatment, he risked his life on the battlefield. However, his days were filled with relentless bullying from his comrades.

One day, a never-before-seen Carrier appeared, leaving Kaito in a hopeless, life-threatening situation…!!

As he lay on the verge of death, unwilling to give up on life, a mysterious girl suddenly appeared before him—effortlessly cutting down the Carriers.

With a fearless smile, she spoke,

“Become my host.”