A legendary mercenary, feared as a weapon of mass destruction on the battlefield, has arrived in Japan. Carrying a new name, Nogawa Ryusei――. His mission is to "raise a baby." However, Nogawa, who knows nothing of a peaceful life, struggles greatly with his first experience of child-rearing!? For the healthy growth of the energetic Seina-chan, eliminate the impending dangers "legally"! A daily parenting tale taken on by a former strongest mercenary!