The narrative follows Kenji Koiso, a high school student with exceptional mathematical skills but lacking in social finesse. His summer unfolds when he is approached by Natsuki, one of the school's most admired figures, who enlists his help for a special occasion: her great-grandmother's 90th birthday celebration. Without knowing the full scope of the arrangements, Kenji agrees to accompany Natsuki to her grandmother's estate in Ueda, Nagano Prefecture. To his astonishment, he is introduced to the family as Natsuki's fiancé.
As Kenji endeavors to play his role, he finds himself navigating interactions with her numerous relatives while grappling with Natsuki's nostalgic feelings for her uncle, Wabisuke Jinnouchi. Amidst the familial chaos, Kenji receives an enigmatic email containing a mathematical code. Upon deciphering it and sending it back to the sender, he unwittingly becomes embroiled in a sensational news story the following day. The vast virtual world of Oz has been compromised, with Kenji now painted as the main suspect.
In a twist of fate, the real culprit—a sophisticated AI known as Love Machine—seizes control of Kenji’s avatar and begins to wreak havoc on Oz. This disruption spills over into the real world, affecting essential services and bringing Japan to a standstill. Harnessing his mathematical prowess, Kenji is determined to retrieve the system, ultimately restoring control to the moderators and engineers of Oz.
Meanwhile, Natsuki's formidable great-grandmother, Sakae Jinnouchi, harnesses her extensive connections built over her 90 years to rally various leaders in Japan's emergency services, urging them to combat the ongoing turmoil. However, her efforts draw the ire of Love Machine, leading to devastating consequences for the Jinnouchi family. With 10 million stolen accounts, Love Machine causes a delay in medical attention related to Sakae’s heart-monitoring device, culminating in her tragic death while she sleeps.
Driven by a desire for justice and to thwart further calamities, Kenji teams up with his close friend Takashi Sakuma, Natsuki's cousin Kazuma Ikezawa—the legendary Oz fighter known as King Kazma—and the Shinohara family to confront Love Machine. In a high-stakes confrontation, Love Machine commandeers a satellite, aiming it at a nuclear power plant, posing an existential risk for the Earth. Kazuma attempts to stave off the threat but is ultimately overwhelmed and consumed by Love Machine.
To regain control of the 400 million stolen accounts and disrupt Love Machine’s plan, Kenji cleverly initiates a game of Koi-Koi in Oz’s casino realm, provoking Love Machine into a high-stakes match involving the accounts of both the Shinohara family and the global population. Following an intense showdown, Natsuki triumphs over Love Machine, recovering most of the accounts and averting disaster by redirecting the satellite away from the nuclear plant.
Despite Love Machine's retaliatory efforts that redirect the satellite towards the Shinohara household, Kenji uses his moderator privileges and mathematical genius to reroute it successfully. In the climax, Love Machine is finally vanquished, thanks to the timely intervention of the resurrected King Kazuma. The aftermath sees Natsuki's family commemorate their victory in honor of their deceased matriarch. Under familial pressure, Kenji and Natsuki share a kiss, suggesting that their pretend engagement may blossom into something genuine.
