Koiso Kenji, a eleventh-grade math genius who works in the summer as a part-time computer technician for the Virtual World of Oz, is invited by Shinohara Natsuki, the most popular girl at his high school to her expansive family's home for a summer job. In the middle of their trip, a hacker infiltrates the network and takes control of Oz, plunging humanity into absolute chaos.

[Source: AniDB]