"Ryuugajou Nanana no Maizoukin" is set on Nanae Island, an artificial island created for students to live and study independently. The island is governed by a unique system where students earn points through academic and extracurricular achievements, which can be used to purchase goods and services. The story centers around Juugo Yama, a high school student who is disowned by his wealthy family and sent to the island to fend for himself. Upon arriving, Juugo rents a room in a low-cost apartment, only to discover it is haunted by the ghost of Nanana Ryuugajou, a cheerful and eccentric girl who was murdered ten years prior.

Nanana, the ghost, was once the leader of the "Great 7," a group of adventurers who explored the island and amassed a collection of treasures known as the "Nanana Collection." These treasures possess extraordinary powers, and their locations are hidden across the island, protected by intricate traps and puzzles. Nanana's murder remains unsolved, and her ghost is bound to the apartment, unable to move on until her killer is brought to justice. Intrigued by the mystery and the potential rewards, Juugo decides to search for the Nanana Collection, aided by Nanana's guidance.

As Juugo delves into the treasure hunt, he encounters other students with their own motivations for seeking the collection. Among them is Tensai Ikkyuu, a self-proclaimed detective with exceptional deductive skills, and Daruku Hoshino, a skilled martial artist. Together, they form a team to uncover the treasures while navigating the dangers of the island. However, they are not alone in their quest; rival treasure hunters and shadowy organizations also seek the Nanana Collection, leading to conflicts and alliances.

The narrative intertwines the treasure hunt with the mystery of Nanana's murder, gradually revealing clues about her past and the circumstances of her death. Juugo's journey is marked by his growth from a disinterested outcast to a determined adventurer, driven by a desire to uncover the truth and honor Nanana's legacy. The series balances action, mystery, and lighthearted moments, with Nanana's playful personality providing a counterpoint to the darker elements of the story.

The island itself serves as a dynamic setting, with its labyrinthine structures and hidden secrets adding depth to the plot. The treasures of the Nanana Collection are central to the narrative, each with unique abilities that influence the characters' strategies and decisions. As the story progresses, the stakes rise, and the true nature of the island and its treasures is gradually unveiled, culminating in a climactic confrontation that ties together the threads of the mystery and the treasure hunt.