From the game box:
The destruction of Halo slowed the covenant, but did not stop them. Fueled by faith, fury and vengeance, they have swept aside all of our defenses and gained a foothold on the most precious prize of all - Earth. With the human millitary scattered and crushed, the fate of the planet lies in the hands of one man, Spartan 117 - the Master Chief. But resourceful and tenacious as he is, the Master Chief will have to rely on new weapons, unexpected allies and a secret so incredible, it will change the Galaxy forever.