A black company employee in his 40s, Namba Kazu, died of overwork and was reincarnated to another world. After saving the world, he returns to find himself regressed to his 17-year-old self. Kazu realizes that he can use the abilities he acquired from another world and is excited to restart his life over again. However, through the help of his childhood friend, Yui, he learns of the existence of human-eating abomination called Variant in the world he was born in. Kazu knows from his past memories that Yui will die in the near future. In order to save Yui, who is on a mission to fight against variants, Kazu decided to throw himself into battle in this world as well.