Amazing gameplay very good sequal to the first Halo, and a exellent prequel to Halo 3.
As you progress through Halo, you find that the Covenant have inadvertently released a parasitic race called The Flood from the Halo ring, a giant ring-shaped weapon that the Covenant intend to use to turn the tide of battle. The Covenant and humans alike are overrun by the voracious Flood and things look grim for both sides. The Master Chief and his AI construct sidekick Cortana begin to look for a way to destroy the Flood. An AI Construct named 343 Guilty Spark from Halo ring tells the Master Chief that the only way to destroy the marauding Flood is to activate the rings s true power. Thankfully, moments before that power is unleashed, Cortana figures out that the true power of the Halo is to wipe all organic life from the universe. There has to be another way.
Cortana and the Master Chief decide that they must strike quickly at the Halo itself to destroy the Flood before they have been given a chance to spread. Using the engines of the Pillar of Autumn battlecruisers wreckage the Master Chief is able to destroy the Halo and the Flood, but not before learning the awful truth here is not just one Halo. There are more.
Halo 2 begins shortly after the destruction of the first Halo. The Master Chief and Cortana have rushed back to Earth to warn them of the impending Covenant assault. They have arrived too late.