From Kodanclub:
A mysterious boy lies naked and unconscious on the street. The story begins when ninth-grader, Ai Oomori, and his third-grader brother, Seigi, find him. This boy has no memory of the past and doesn't even know his real name. He is like a little baby. Ai names this boy Makoto and takes him back home.
Makoto has superhuman physical powers and can even recover quickly from injuries. Who is Makoto? Makoto is easygoing and not at all bothered by everyone's curiosity. Until that is he is pursued by a cruel woman who disguises herself as a spider. A neo sapiens, this spider woman has been dispatched by a mystery organization called, "Love & Peace." Her mission is to bring back Makoto dead or alive.
Makoto survives the spider woman's attack when she eventually finds him. Indeed, he is the "Last Man," created to be mankind's last survivor and future hope on earth. What is this dreadful plan that Makoto has gotten involved in and what is "Love & Peace" up to?