Dark Sector thrusts players into the role of Hayden Tenno, a covert operative sent on a dangerous mission into Lasria, an Eastern European city on the brink of ruin that hides a deadly Cold War secret. In the midst of this mission, Hayden is attacked by an unknown enemy and infected by the Technocyte Virus, a brutal bio-weapon that twists its victims into mindless killing machines. Surviving this attack, Hayden finds that the Technocyte virus has granted him powerful, inhuman abilities unlike anything he has ever seen. Now, he must evolve with his powers, survive and become a hero.

Near the end of the Cold War, a sunken submarine off the coast of the country of Lasria releases a mysterious and horrifying infection known as the Technocyte, which spreads mutation and destruction before the Lasrians manage to bring it under control. Years later, covert operative Hayden Tenno is sent deep into a grim Lasrian gulag compound where the ruthless Robert Mezner is holding those infected with the virus. Hayden's orders are clear: locate his target, halt the spread of the Technocyte, and eliminate Mezner. But the mission goes catastrophically wrong. Hayden is confronted, overpowered, and deliberately infected with the very virus he came to contain. As the Technocyte takes hold and transforms his right arm into a monstrous weapon, Hayden must fight for survival while grappling with the dark power growing inside him.