Shinnosuke and his grandfather reside in the forest of Ryugenzawa, where they safeguard the local villagers from giant animals that were once their pets. Many years ago, while Akane was on a family vacation in Ryugenzawa, she got lost in the forest and was attacked by a giant platypus. Fortunately, Shinnosuke came to her rescue. Sometime later, Shinnosuke nearly died, but his grandfather saved him by administering some of the Water of Life. Although Shinnosuke survived, he became dependent on the Water of Life.
In the present day, Akane learns about the troubles in Ryugenzawa through a television advertisement asking for help to combat the increasing number of monsters. She recalls how Shinnosuke saved her when she was young and decides to travel to Ryugenzawa to repay his kindness. During her visit, Akane is once again attacked by a giant creature and is rescued by Shinnosuke, who still bears the scars on his back. Despite his forgetfulness, Shinnosuke invites Akane to stay with him and his grandfather. Eventually, Akane helps Shinnosuke overcome his dependency on the Water of Life by applying a special moss to his back.
