Yeah. Long story short, Master Chief woke up on his space ship (cryo sleep) to find it under attack by the aliens (Covenant). He crash lands on a Halo ring. Halo rings are big weapons capable of destroying all life in the galaxy. There's a virus-based race called the Flood. By killing the Flood's food (sentient life) the Halo rings can destroy the Flood. That's why they were built. Covenant are religious fanatics who thing the ring will bring them to God, basically. You killed a bunch of aliens, made friends with one of them and the humans and Covenant are now at an uneasy peace by treaty. Master Chief ended up on a half-broken space ship at the end of part III and this game starts w/ him waking up something like 4 years later.