On his way home from work late one night, the workaholic Dr. Jikumaru Shuuji falls through a mysterious hole and wakes up in... another world! There, he stumbles upon the nearly dead assassin Naina and saves him with his otherworldly "healing powers," but come morning, he discovers that for some reason, he now looks like a child!? Jik (Jikumaru) starts looking for a way to return, while Naina protects him from this strange new world where the only law seems to be the law of the jungle. While Jik is busy with that, though, he starts to feel that maybe living in this world together with Naina wouldn't be so bad... but is that possible? ワーカホリックな医師・軸丸柊治は夜勤明けの帰宅途中で謎の穴に落ちてしまい、着いた先は・・・異世界だった！！ たまたま見つけた瀕死状態の殺し屋・ナイナを異世界お約束のチート治癒力で助けるも、一夜明けるとなぜか自らは幼児の姿に・・・！？ 弱肉強食が日常の異世界でナイナに守られながら元いた世界へ帰る方法を探す旅をするジーク（軸丸）だったが、いつしかナイナとともにこの世界で生きたいと気持ちが変化していったが・・・？