A time in the near future… People now depend on the computer-simulated virtual reality world of "OZ" for many aspects of their livelihood, from shopping to playing games, communication means, and administrative procedures.

One summer, Kenji, a high school student who works as a part-time moderator for OZ with his friend Sakuma, is invited by Natsuki, a senior classman who he has a crush on, to her hometown in the countryside for a "side job." Natsuki is from Nagano Prefecture, and her family, descendants of the famous Jinnouchi samurai clan, is headed by her great-grandmother, Sanae, and comprises many lively, individualistic members. Kenji's so-called side job is to pretend to be Natsuki's fiancé in front of her relatives.

One evening, Kenji receives a text on his cell phone containing a code-like series of several hundred numbers. Kenji, a gifted mathematician, stays up all night and cracks the code. The next morning, they find out that OZ's infrastructure has been hacked, and the real-life world and the world of OZ fall into great chaos. At first, Kenji is suspected of being involved in the breach, but they later find out that the real perpetrator is Love Machine, an artificial intelligence that exists in the Internet. To make matters worse, Love Machine was written by Wabisuke, the family troublemaker. Although Wabisuke had designed Love Machine solely to win Sakae's approval, Sakae and the other family members condemn him for putting society in such a huge predicament, and he disappears from the family estate.

Thanks to Sakae's wide network of friends and associates, the situation seems to come under control. However, Sanae suddenly dies of a heart attack, and Love Machine's devious wagers continue to escalate. As they mourn for Sanae, Kenji and the Jinnouchi men, including Kazuma, a battle game champion in OZ, challenge Love Machine to a match, but they lose. Then Love Machine retaliates by redirecting an asteroid probe onto a collision course with a nuclear power plant somewhere in the world. The countdown to the catastrophe starts.

"We haven't lost yet."

Natsuki and the family are encouraged by Kenji's words and Sanae's will. Natsuki also convinces the family to bring Wabisuke back into their lives, and he also joins the fight.

And thus starts the final battle between the Jinnouchis and Love Machine, a showdown that will determine the fate of the world.