In 2xxx, a meteorite collided with Earth. Along with the severe damage coursed by the impact, a toxic gas was released from the meteorite which almost led to the extinction of most life forms on Earth including most of mankind.
Those lucky enough to survive the disaster suffered from mutations due to the virus from the toxic gas.
People closer to the impact were more heavily effected by the virus and mutated into ferocious creatures with super human strength.
After years of war between the survivors, only two factions remained, waging constant battle against each other. On one side stood the ”Government of Gravity” which possessed most of Earth's resources on and on the other side, the fierce and ruthless ”Axis of Evil”.

During the continuous wars between the two sides, the Government of Gravity constantly found themselves outnumbered and struggling to keep up with its enemy's large number of troops. The Government of Gravity decided to hire groups of mercenaries from over all the world - among them the group known as D Team was the strongest. All members of the D Team came from different backgrounds and joined the Government of Gravity to protect their homes and countries. For the safety of their beloved ones, they had to conceal their identities behind masks and chose a Hanzi (Chinese character) to replace their real name.

This is the debut of our second main character - Sha, one of the D Team members and a former shark hunter. Due to his facination of sharks, the strongest predator of the ocean, he named himself Sha (Chinese word for Shark).
The thruster on his underwater gear was made of scrap mechanical parts but was unfortunately damaged during one of his earlier shark hunts and wasn’t repaired due to lack of resources. Only after Sha joined the D Team, the thruster was finally fixed by the technicians and was even upgraded with extra propulsive force. His weapon of choice was a maguro bōchō, an upgraded and enlarged tuna cutter originally used to dissect tuna.
Since Sha was at the other side of the Earth at the moment of the meteorite strike the virus only had little influence on him. The only difference between him and other people is his superhuman speed.