Halo: Combat Evolved
Halo: Combat evolved is the first game in the Halo series and the one that started it all.
The game tells the story of the UNSC (United Naval Space Command... basically humanity) and it's struggle for survival in an intergalactic war against The Covenant. More specifically the tale follows the last human super solder; called "Spartans" and his struggle to survive an encounter with overwhelming Covenant forces.
The story begins with the fall of the planet Reach in the background. Planet Reach was the largest and one of the last human military colonies not in Earth's home solar system. At this point in the story The Covenant have been systematically eradicating all human life they encounter on their inexorable march towards Earth and the destruction of mankind.
The only ship to escape the destruction of Reach is a Halcyon-Class cruiser by the name of The Pillar of Autumn. On board in cryo-stasis is the last Spartan II, Master Chief John-117. In its frantic attempt to escape the AI (Cortana) controlling the ship does a random slipspace jump and when the ship exits slipspace it arrives at a huge habitable ring in space created by The Forerunners called Installation 4 and it is under Covenant control.
Under heavy fire from several superior Covenant ships the Captain isses an abandon ship order, turns the AI over to Spartan 117 and once all hands are in the life pods, crash lands the Pillar of Autumn on Installation 4.
Spartan 117 survives his descent to the surface and battles the Covenant all across the ring in a battle for control of the installation that turns out to be a massive intergalactic weapon capable of destroying all sentient life in the galaxy.
During this battle a super virus called "The Flood" is released from quarantine. The dead rise and the horrible infected Flood join the battle for control of the installation.
It is revealed during the battle that the Forerunners built Installation 4 and six others (called the Halo Array) like it to study, control and ultimately destroy the Flood. The Covenant belive the Forerunners to be gods and worship their technology. They believe that firing the rings will allow them to ascend to godhood as well. As the Covenant fights to control and ultimately fire the Halo Array the Flood and Spartan 117 fight to stop them for different reasons.
Eventually Spartan 117 has no choice but to overload the reactos on the Pillar of Autumn desstroying Installation 4 and barely escaping with his life and the AI he was entrusted with intact.