The story centers on high school student Kenji Koiso, a wiz in mathematics but ill-adept socially, who spends his time in OZ, a massive computer-simulated virtual reality world, as a part-time low-end moderator. At the beginning of summer vacation, he is asked by Natsuki, one of the most popular girls in his school, to help in celebrating her great-grandmother's 90th birthday. Although Kenji does not know the details of the plan, he agrees and goes with Natsuki to her grandmother's estate in Ueda, Nagano Prefecture. Much to Kenji's surprise, he is introduced at the Shinohara family house as Natsuki's fiancé.As Kenji tries to act the part, meets over a dozen of Natsuki's relatives, and deals with Natsuki's "first love" crush on her uncle Wabisuke Jinnouchi, he receives a mysterious e-mail with a mathematics code. He cracks the code and returns it to the sender but finds himself at the center of a developing news story the following day. Overnight, OZ has been hacked and he is the prime suspect. Using Kenji's account and avatar, the real "hacker", which is actually the advanced artificial intelligence system Love Machine, wreaks havoc on the virtual world of Oz which is affecting the real world that is deeply ingrained into Oz' system. With car navigation systems disabled and computer systems malfunctioning, Japan grinds to a halt. With his math skills Kenji is able to crack the passcode and returns control of Oz back to the moderators and Oz' engineers. Meanwhile Natsuki's great-grandmother, Sakae Jinnouchi, uses the level of influence she's created in her 90 years of life and calls various people in important positions in Japan's society and her family who work in various emergency services. She encourages them all to work their hardest, to see through this troubling time of turmoil, reducing the potential chaos and damage that could have occurred. But her actions do not go unnoticed by Love Machine, which leads to an unfortunate tragedy for the Shinohara clan. Although no longer controlling Oz itself, Love Machine continued to steal user accounts, numbering over 10 million at the time, with one being linked to a heart monitoring system for Natsuki's great grandmother. This causes delayed care and ultimately her death when her heart fails her in her sleep. In order to prevent similar incidents and get revenge, Kenji enlists the help of fellow mod and best friend Takashi Sakuma, Natsuki's cousin Kazuma Ikezawa (who is the legendary Oz fighter King Kazma), and the rest of the Shinohara family to defeat Love Machine.After escaping from a trap and winning a combat online, Love Machine takes control of a satellite and sets it on a crash course with a nuclear power plant somewhere on Earth where the fallout could destroy life on the planet. Kazuma tries to fight Love Machine again, now embodied in over 400 million stolen accounts as an almost living botnet, but he is no match and is absorbed by Love Machine. Needing a way to take back the 400 million stolen accounts and find the one linked to the satellite and its GPS, Kenji uses the fact that Love Machine sees everything as a game and has Natsuki's family provoke Love Machine into a game of Koi-Koi in Oz' casino world. There they wager the accounts of the Shinohara family and eventually that of the people throughout the world. Natsuki defeats Love Machine and manages to regain almost all the accounts stolen and in the process stopping the satellite from crashing into the nuclear power plant. However, Love Machine is able to set the satellite navigation and in revenge directs it toward the Shinohara household instead. Using his moderator level of access to Oz and math skills, Kenji manages to hack into the GPS of the satellite and redirects it away from the house, while Love Machine is deleted by the newly rescued King Kazuma. In the aftermath, Natsuki's family celebrate the victory in the memory of their late grandmother. Under the pressure of Natsuki's family, who suggest that the two should marry for real after lying to the family about their engagement, Kenji and Natsuki kiss. [wikipedia]