In the beginning of 21th century, technology has evolved in unprecedented ways. But so did crime. Huge corporations, which were in fact criminal organizations, came to control the world. Finally, a mega-corporation known as Connection has taken the power in its hands. Everything was sponsored with its money; the governments and the law were silent. Only vigilante groups called "Connection-Killers" still tried to resist. In 2015, in Detroit, one of those people, nicknamed DD, witnessed the death of his comrades...

A year has passed. DD moves to Brain City - a highly developed, modern city, the center of scientific research. It was built on an island in the Atlantic Ocean, which appeared after a volcano eruption. It is controlled by Connection. DD opens an investigation office in the city, trying to get to the heart of Connection and defeat his old nemesis, despite the hostile atmosphere and the grave dangers that await him there...

His Name is Diamond is a RPG with top-down navigation and first-person turn-based combat. The player controls DD as he moves through the urban environments of the game. The enemies appear randomly and are mostly grotesquely exaggerated everyday people (workers, journalists, etc.), with a large amount of sexy women. DD deals with the enemies through various menu commands. Instead of a simple "attack" option, there are various techniques and extensive item usage, so that many of the fights feel almost like mini-puzzles. DD gains experience and levels up, increasing his parameters, after defeating enemies.