At Kamome Academy, rumors swirl around the school’s infamous Seven Mysteries. The most famous of these is Hanako-san of the Toilet, a ghost said to haunt the third stall of the third-floor girls’ bathroom in the old school building who will grant any wish to the person who successfully summons her. Hoping to improve her own romantic fortunes, a lovesick first-year girl named Nene Yashiro decides to test the legend. However, when she knocks on the stall door, the spirit that appears is not a girl at all, but a knife-wielding boy who introduces himself as Hanako-kun.

Nene quickly learns that her wish comes with a price. After a mishap involving a mermaid’s scale, she is cursed to turn into a fish. To save her, Hanako swallows a scale as well, binding their souls together and transforming Nene into his new human assistant. As Hanako-kun’s assistant, Nene discovers his true duty is to maintain the fragile balance between the human world, known as the Near Shore, and the spirit world, or the Far Shore. Together, they encounter and correct the other supernatural mysteries of the school, altering dangerous rumors to keep both students and spirits safe.

Along the way, they are joined by Kou Minamoto, a kind-hearted but inexperienced boy from a famous family of exorcists. Though Kou initially tries to exorcise Hanako, he soon realizes not all supernaturals are evil and becomes a close ally. The trio faces various supernatural threats, including the Misaki Stairs, a wonder that traps its victims in an alternate reality. The most emotionally charged arc of the first season involves Mitsuba, a ghost who was a classmate of Kou’s. Mitsuba, who died in an accident, desperately wished to have friends and be remembered. After being manipulated by Hanako’s mysterious and sinister twin brother Tsukasa, Mitsuba turns into a violent supernatural, forcing Hanako to destroy him in a moment of profound tragedy that leaves Kou devastated.

Beneath the show’s lighthearted and comedic surface lies a deep mystery. Hanako is revealed to be the ghost of a boy named Amane Yugi, who died decades ago under dark circumstances involving the murder of his twin brother, Tsukasa. He carries a kitchen knife as his weapon, a direct symbol of the sin he committed in life. As the leader of the Seven Mysteries, he has been promised absolution for his crime if he performs his duties, though he remains haunted by guilt. Throughout the series, Nene begins to uncover hints of Hanako’s tragic past while also discovering a terrifying truth about her own future: her lifespan is rapidly shortening, and she only has one year left to live, a secret Hanako already knows and desperately wants to change.