From Viz:

It is the middle of the 21st century and Tokyo is expanding more and more, filling Tokyo Bay with landfill to make room for the bursting population. But suddenly Tokyo suffers one of its greatest earthquakes, and the landfill liquefies, creating untold damage in the newly-made urban area. The members of Self-Defense Force Rescue Squad Four are sent to help. But what they dig up is a plot of murder and destruction--one that wipes out nearly the entire squad and leaves its two surviving members, the fierce Rei and her cool, handsome partner Uozumi, in a fight for their lives.

There is only one place where they can find the work that keeps them alive, and the support so they can start uncovering the mystery that killed their squad: that's in the south-Shinjuku bar called Chicago. As the earthquake levels Japan's largest city and capital and leaves countless dead, rescue workers, Rei and Uozumi, search for survivors amidst the rubble of Bay District D only to find a mysterious string of dead bodies--bodies with bullet holes. Without warning, the pair is under fire from the air as a hail of gunfire rains down. Are these the killers, and if so, what is their purpose? Barely escaping with their lives, Rei and Uozumi discover that their squad is listed only as "collateral damage" and their records wiped clean.