D.U.O. ~song for all~

December 25, 2001 AD. The first year of the 21st century was about to pass in an atmosphere of joy. Yet it was precisely at this moment that the greatest calamity in human history—the "Virus Crisis"—descended. A disease never seen before spread across the globe, and two-thirds of the world's population died in it. Because nothing like this had ever happened in human history, people speculated that this mysterious infectious disease (or so-called biogenic crisis) was a natural phenomenon of human culling. Those who survived the calamity decided to stand together and strive ceaselessly for the revival of human society. Nearly a century passed...
Ironically, the problems that had once given everyone headaches back then (in 2001)—population explosion, environmental pollution, and so on—no longer existed because of that calamity. The remaining people focused on the development of scientific power and had come to realize their past mistakes. As time went on, humanity once again reached the pinnacle of prosperity.
The stage of this story is Tokyo, Japan, in the year 2099.
Thanks to continued prosperity, Tokyo attracted people of all kinds of ethnicities to settle there, and the barriers between races gradually diminished through fusion. Everything was so wonderful... and then, a series of serial deaths occurred. The victims all shared something in common: they all died when a virus invaded their central nervous systems, yet no one knew how the virus had entered them. Who did this? Who would be the next to die? The public disclosure of this matter made everyone recall the catastrophe of 2001 once again... There were far too many similarities between them...
Fearing that public panic would affect the stability of society as a whole, the authorities decided to conduct a detailed investigation of the entire incident. In this era when even security institutions had been privatized, the actual executors of criminal investigations were the so-called "private detectives"...
In the end, the authorities chose the protagonist of this game, "Takuya Takabashi," a private detective renowned for his ability, to take charge of the case. At the time, Takuya had nothing to do because he had no business, so he agreed to the authorities' request without hesitation. But things were not that simple...
Takuya's detective agency is on the second floor of a building in Shinjuku; on the first floor is a bar called "MEMROIES." This bar is a music bar run by Takuya's adoptive father, Ichiro Takabashi. As for the staff, besides Ichiro Takabashi's daughters Hitomi and Aimi, there are several others. Hitomi and Aimi generally help out at Takuya's detective agency in the morning and sing at their father's bar at night. Takuya and the two sisters are very close, just like real siblings. In fact, the protagonist Takuya is an orphan who was picked up by Ichiro Takabashi before he was old enough to understand things, so Takuya knows nothing about his own birthday or his original name.
Centered on Takuya Takabashi, the relationship between this story and the catastrophe of 2001 will gradually unfold...