Yama Juugo is a high school boy who has moved to Nanae Island, also known as the Special Student Zone, after being kicked out of the house by his father. He has chosen to begin his solo life in room 202 of the Happy House, a one-room apartment plus kitchen with separate bath and toilet, including furniture, appliances, and extras for 5,000 yen per month. But when he enters the room, he sees a girl relaxing inside, watching TV...

Juugo picks up a jewel that fell from the sky as he was on his way home from the convenience store. It turns out to be one of Nanana's Collection: the The Eye of Yuguthos. Juugo is shocked that Nanana's Collection really exists. Meanwhile, the master detective Ikkyuu Tensai and her assistant, Hoshino Daruku, pay a visit to room 202 of the Happy House in search of the stolen jewel.

After clearing the ruins in the classroom, Juugo, Tensai, and Daruku become members of the Adventure Club. At club president Yuiga Isshin's request, Juugo undergoes a trial administered by Nanana to get her to share the location of the next treasure. This complete, the Adventure Club sets off to the Rising Building. As they search for the ruins, Tensai quickly finds a certain door...

After Juugo and Tensai escape from the Rising Building, Juugo receives an email from Tensai saying that Isshin plans to sell the Wizard's Cane to Ikkaku Shunjuu.

The Adventure Club is back in action. Isshin proposes that they plan an overnight trip to a hot springs town on the west side of the island, where he suspects they'll find the next ruins.

The Adventure Club has come to a hot springs town. In the hallway of their inn, Juugo sees Yukihime with a friend of hers, but she ignores him. That evening, Isshin shows everyone the layout of the nearby house that he has brought them to explore, and Tensai pinpoints the room that the ruins are in.

Juugo has been cowering in fear in a corner of his room while Nanana has been on the rampage. Finally the delivery he's been waiting for arrives, and he presents Nanana with its contents, special thick pudding from the hot springs town, along with an apology for selling off her video games to pay his utility bill. But there was something else hidden in the package...

At the start of class one day, a first year student comes to see Tensai. After speaking with her, Tensai returns in a bad mood and walks out of the class. The same day, one of the people Juugo met in the Tsukuyomi District, Tetsu, transfers into Third High School.

Tensai and Daruku have not been seen since meeting with Suruga Aya. Though he's worried about them, Juugo goes out to have fun at Southred Mall with Yumeji, Tetsu, and Yun. Ikusaba Hiiyo has been watching them strengthen their friendships and follows them, while Tsurezure Kagetora follows him.

Hoping to clear the ruins in Southred Mall before Ikusaba, the Adventure Club follows Nanana's hints to find the entrance to the ruins. But the entrance is too visible to passers-by, so they agree to come back that night to conquer it. Back at the Happy House, Nanana asks Juugo several questions, but Juugo is too anxious about Ikusaba to talk.

Isshin's plan has rendered Ikusaba's "Shadowfins" useless, and victory is right before the Adventure Club's eyes... but then Ikusaba unleashes the power of his other Nanana's Collection treasure, leaving Juugo in serious trouble. Then Tensai steps in, telling Ikusaba she's figured out his power and offering him a proposal.