Within Nagasaki City's Shizundamori, living within the confines of a wrecked warship, lives what many consider to be no more than a legend, an entity built around cautionary tales. To every member of the Matsuyama household, the fabled swamp wardens are very real. The mother, Haru, holds a disdain for them, while the children, Kurisu and Olivia, share oblivious intrigue. With an encounter on his balcony one night, Kurisu, an eighteen year-old boy, has his intrigue lead him to this swamp warden, where he would unknowingly lay the groundwork for a sudden friendship that will whisk him and Olivia away on a fairy tale-esque odyssey across seas, meeting countless people of interest, and learning more about the swamp wardens than any oral tradition could ever teach.

Kurisu Matsuyama, a boy eager to know. Growing up, swamp wardens were nothing more than fictional entities made to scare people out of sacred forests at the request of the great sea kami, Umi No Shika. Come one August night, this shinto legend would manifest itself on his bedroom balcony. A lonely swamp warden of no name intrigues him, brining him back to the tales of his childhood, those now unfortunately tied to his father's disappearance. Kurisu finds himself searching for, and finding the lonely warden, a young woman about his age. The mother, Haru Matsuyama, knows the painful truth. Kurisu, now empowered with his love and sympathy for the warden now named Jayda, and the human urge to know more, sets out from Nagasaki City with his kid sister clinging to his back. Meeting an unexpected ally facilitates them in their escape and pursuit of knowledge. Jayda, alongside Kurisu, is further educated about the swamp wardens, and gradually, more of Jayda and her people's tragic story is revealed. Crossing the sea takes them to an island laden with many surprises, turning their exploratory excursion into something far more urgent. Many friends, and just as many foes await our trio, some ascending far above the mortal plane.