From Steam:
In Warhammer® 40,000® Space Marine® you are Captain Titus, a Space Marine of the Ultramarines chapter and a seasoned veteran of countless battles.

A millions-strong Ork horde has invaded an Imperial Forge World, one of the planet-sized factories where the war machines for humanity's never ending battle for survival are created. Losing this planet is not an option, but a darker and far more evil threat is lurking large in the shadows of this world.

With an Imperial liberation fleet en-route, the Ultramarines are sent in to hold key locations until reinforcements arrive. Captain Titus and a squad of Ultramarine veterans use bolter and chainsword to take the fight to the enemies of mankind.