Saisei Man centers on the lives of Hitomu Abe and Mirai Kaiyō in a world populated by superheroes. Living in the run-down city of Kyūkei, Hitomu wants nothing more than the world to see him, a wish which will never be fulfilled in his boring office job.
However, after an attempted scuffle is observed as a good deed by beings above our world, Hitomu is gifted with the power of complete regeneration. The catch? Every injury inflected upon to him is transferred to another citizen of Kyūkei.
Meanwhile, Mirai Kaiyō possess the ability to comprehend and manipulate "thread theory", which gives her limited but powerful control over time. With mask-adorned adventuring being the only constant in her life, she makes it her mission to find the recently-appearing "invisible hand" killer before he leaves the city in complete disarray.