Mercenary is set on the planet Eris, which is under threat from a comet named "Damocles". The comet is on a collision course with the planet and has only a few hours to spare before it's too late. Your task is to find a way to stop the comet and save the planet.
The game features non-linear gameplay, allowing you to explore the galaxy, visit any of the 9 planets or 19 moons, and complete objectives in your own time. You can walk on planetary surfaces, drive vehicles, and fly spaceships, using objects and examining buildings to progress through the game. Your goal is to prevent Damocles from destroying Eris.