A high school student named Jugo Yama is disowned by his father and sent to live on Nanaejima, a vast artificial island built in the Pacific Ocean as a Special Student Zone filled with schools and research facilities. With only a small monthly allowance, he rents a room in a low-rent apartment complex called Happy House for only 5,000 yen a month. Room 202 is incredibly cheap for a reason: it is haunted by the ghost of Nanana Ryugajo, a beautiful 18-year-old girl who was murdered in that room ten years ago and is now bound to it as a geister. Rather than being a terrifying specter, Nanana spends her days as a cheerful NEET (Not in Education, Employment, or Training), passing the time by playing video games, surfing the internet, and eating her beloved pudding.

Jugoo quickly learns that Nanana was no ordinary girl. In life, she was a legendary adventurer and the leader of the Great 7, a group of seven prodigies who provided the funding and vision to create Nanaejima. Her adventures across the globe amassed a vast fortune, but her most valuable acquisitions are the Nanana Collection, a set of artifacts that possess extraordinary and mysterious powers. After her death, these treasures were hidden across the island inside complex, trap-filled locations known as ruins. The person or group known only as Leprechaun scattered them, turning the island into a massive, high-stakes treasure hunt royale.

Determined to help Nanana find peace so she can move on, Jugoo resolves to uncover the truth about her murder. To this end, he joins the school's Adventure Club, a group dedicated to exploring ruins and recovering the Nanana Collection. He is joined by his eccentric neighbor, Tensai Ikkyu, a self-proclaimed master detective with brilliant deductive reasoning skills but a bizarre and arrogant personality. Serving as her assistant is Daruku Hoshino, a soft-spoken and skilled individual who is biologically male but presents and dresses as a maid. Together, this unlikely team navigates deadly mechanical puzzles, outmaneuvers rival treasure hunters, and contends with the island's underground organization, the Three Skulls.

The hunt for the collection brings them into conflict with several major factions. The Adventure Club itself, led by the ambitious and sometimes treacherous third-generation president Isshin Yuiga, has its own agenda. A more sinister threat comes from Hiiyo Ikusaba, the ruthless former vice-president of the club who believes in achieving his goals by any means necessary, even if it means sacrificing his own comrades. Jugoo also carries a secret from his past: he is the heir to Matsuri, an ancient organization of noble thieves who claim to steal only from the wicked. He was banished to the island for refusing to take over the family business, but his upbringing gave him exceptional thieving and combat skills. Members of Matsuri, including his childhood friend and mentor Yukihime Fugi, are also on the island collecting the treasures, blurring the lines between ally and enemy.

Notable narrative arcs in the series focus on the race to acquire specific powerful artifacts from the Nanana Collection, such as a cane that can grant wishes and a jewel that reveals lies. These arcs explore the characters motivations, with Tensai seeking the thrill of solving puzzles, Isshin dreaming of conquering the world, and Jugoo wrestling with his own cynical belief that helping others is foolish while he dedicates himself entirely to Nananas cause. A central, overarching mystery concerns the identity of Nananas killer, a secret that some of her old friends from the Great 7 may know but are terrified to reveal. As Jugoo and his friends collect more artifacts, they draw closer to the truth, unearthing clues about the conspiracy that led to her death and the powerful forces that may want to keep it hidden forever. The story balances comedic cohabitation and lighthearted banter with genuine dramatic tension, culminating in a confrontation with the powerful underground boss Saki Reijou and a deeper dive into the founding mysteries of Nanaejima itself.