A boy named Juugo Yama is disowned by his father and sent to live on Nanaejima, a man-made "Special Student Zone" island filled with schools and research facilities. With only 40,000 yen a month to live on, he rents a suspiciously cheap apartment in the Shiawase Sou building, only to discover it is already occupied by the ghost of Nanana Ryuugajou, a beautiful young woman who was murdered in that very room ten years ago and has been bound to it ever since. Unlike a typical specter, Nanana is a cheerful and eccentric NEET who spends her eternal afterlife playing video games and eating pudding.

The island of Nanaejima was originally funded and developed by a group of legendary adventurers known as the Great 7, of which Nanana was a member. During her life, she accumulated a vast collection of artifacts and treasures from around the world, many of which possess strange and mysterious powers. After her death, this collection was scattered and hidden across the island by an unknown group called the Leprechauns, becoming the legendary "Nanana Collection" that everyone seeks.

Intrigued by his ghostly roommate and her unsolved murder, Juugo joins his school's Adventure Club, an organization dedicated to hunting for the Nanana Collection. The club is filled with colorful members, including the brilliant but socially awkward detective Tensai Ikkyuu, who declares Juugo her rival, and her feminine and gentle assistant Daruku Hoshino. The club's president, Isshin Yuiga, is an ambitious upperclassman obsessed with the collection's power who often proves to be an untrustworthy ally.

Juugo soon reveals that he is not just a disowned son but the heir to Matsuri, an ancient and noble organization of thieves. He was exiled to the island after refusing to take his father's place as leader due to a clash of ideals. Despite this, he uses his thieving skills and a special crystal called the Eye of Yuguthos, which reveals lies, to stay ahead of rivals in the "treasure hunt royale".

The central narrative follows two parallel threads. The primary conflict is the search for the Nanana Collection, which puts Juugo and the Adventure Club in competition with other treasure hunters, including members of the Matsuri like his martial arts teacher Yukihime Fugi, as well as rogue former allies. The second and most personal thread is Juugo's determination to find Nanana's killer so her spirit can finally move on. As he gathers more artifacts, he uncovers clues about the Great 7 and the dark secrets surrounding her death. A major antagonist in this quest is Hiiyo Ikusaba, a powerful and ruthless former vice-president of the Adventure Club who once used a club member as a sacrificial pawn to obtain a treasure and has no qualms about using violence to achieve his goals. The hunt for the treasure becomes increasingly intertwined with the hunt for the truth, revealing that the reason for Nanana's murder may be directly connected to the very collection she left behind.