On the artificial student island of Nanaejima, a paradise built to foster young talent, a beautiful ghost named Nanana Ryugajo spends her afterlife as a NEET, confined to a single room playing video games and eating pudding. Ten years before the story begins, the legendary adventurer was mysteriously murdered, and her spirit has been bound to that apartment ever since. Juugo Yama, a high school student disowned by his father, arrives on the island with limited funds and rents the cheapest room available, only to discover it is already inhabited by Nanana’s ghost.

Juugo learns that before her death, Nanana and her adventuring group, the Great Seven, hid a vast collection of artifacts known as the Nanana Collection across the island. These treasures possess extraordinary and mysterious powers. Determined to help Nanana move on, Juugo resolves to find her killer by collecting these artifacts, hoping one will reveal the truth. He joins the school’s Adventure Club, a group dedicated to hunting for the Nanana Collection, and becomes entangled in a fierce treasure hunt royale against other students, secret organizations, and formidable rivals.

The main cast includes the eccentric self-proclaimed master detective Tensai Ikkyu, who becomes Juugo’s rival and neighbor, and her loyal, cross-dressing assistant Daruku Hoshino. Together, they navigate traps and puzzles set by Nanana herself. Key narrative arcs involve Juugo’s past as the heir to Matsuri, an ancient organization of noble thieves he rejected, which makes him a target and gives him unique skills. The Adventure Club faces antagonists like Isshin Yuiga, the ambitious club president willing to betray anyone for power, and Hiiyo Ikusaba, a brutal former club member who uses violence to claim treasures. Juugo must use his wits and thieving talents to outmaneuver these foes while uncovering the secrets behind Nanana’s death and the powerful relics scattered across the island.