A teenage boy named Juugo Yama is disowned and exiled by his father, forcing him to transfer to a high school on Nanaejima, a massive artificial island in the Pacific Ocean designed as a special zone for students. With a severely limited budget, he is forced to rent a suspiciously cheap apartment in a boarding house called Happy House. He soon discovers the reason for the low rent: the apartment is haunted by the ghost of its former resident, a beautiful and lively girl named Nanana Ryuugajou. She was murdered in that very room ten years prior and is now bound to it, unable to move on to the afterlife. Despite her situation, Nanana spends her days as a cheerful shut-in, occupying her time with non-stop video gaming.

Juugo learns that Nanana was no ordinary girl. She was a famous and wealthy adventurer who provided much of the initial funding to build Nanaejima. Her greatest legacy is the "Nanana Collection," a set of extraordinary treasures she gathered during her travels, each possessing strange and mysterious powers. Before her death, she hid these artifacts across the island, protecting them with elaborate and deadly traps. The search for this collection has become an unofficial but intense pastime for the island's students. Juugo becomes entangled in this treasure hunt after finding a hidden notebook left by a previous tenant, which encourages him to investigate. He decides to hunt for the collection, hoping that finding clues about the treasures will lead him to the truth about who murdered Nanana, allowing her spirit to finally find peace.

Juugo is soon joined by eccentric allies. He meets Tensai Ikkyuu, a brilliant but diminutive girl who proclaims herself a master detective, and her loyal attendant Daruku Hoshino, a boy who always dresses in maid clothing. Together, they are reluctantly recruited into the school's Adventure Club, led by the ambitious and manipulative Isshin Yuiga, who seeks the collection to gain power. The club’s other members include Yuu Ibara, a strong fighter who is fiercely protective of Isshin, and Kagetora Tsurezure, a more laid-back observer. The story follows their competitive and often dangerous hunts for the hidden artifacts, where alliances are temporary, and rival treasure hunters, including members of a secretive thieves' organization called Matsuri, frequently interfere. Juugo discovers he has a personal connection to Matsuri, as he is the estranged heir to its leadership, giving him unique skills that both help and hinder his quest.

Major narrative arcs involve the pursuit of specific legendary items from the Nanana Collection, each requiring the group to solve complex puzzles and survive deadly dungeons scattered across the artificial island. A central arc follows the betrayal within the Adventure Club when they successfully locate a treasure, forcing Juugo and his friends to operate independently. A significant conflict arises with Hiiyo Ikusaba, a dangerous former president of the Adventure Club who possesses multiple powerful collection items and becomes a primary antagonist. Throughout these adventures, Juugo struggles with his own reluctance to commit to helping others, even as he becomes more determined to solve the mystery of Nanana's death. The hunt for the killer intensifies as he faces off against rival treasure hunters, uncovering dark secrets about the island's founding and the circumstances that led to Nanana's murder a decade ago.