In the early 1980s, rumors begin to circulate about a new entertainment medium called “D Movie”, which allows people to immerse themselves in a world of virtual reality. As D Movies gain traction, a trend emerges of disaffected young people taking permanent refuge in the virtual world while abandoning their physical bodies to atrophy. To retrieve these so-called “Sinkers”, Japan's National Public Safety Commission establishes Public Security Division Four, nicknamed “Diamond Dog”, in 1984. The agents who enter the virtual world and perform these rescues are known as Debuggers.

In 1992, a seventeen-year-old girl named Yūki Sayaka sinks into “Story of the Ruined Capital”, a D Movie directed by David Visconti. Three days pass before she is discovered, and combined with her weak constitution, she is expected to die within twenty-four hours. Two Debuggers, Shijima Ryō and Tobari Maria, are dispatched to rescue her before that happens.