Ok, here's the short, short version.
There's a huge battle, human vs Covenant (a collection of alien races), at a planet called Reach, humans lose and the Pillar of Autumn makes a strategic retreat while being chased by some Covenant ships. Knowing they'll lose in a straight up fight, the Pillar lightspeeds in a random direction hoping to lose the Covenant. Doesn't work and the Cov. damage the Pillar forcing it to land on the unknown ring (Halo). You're Master Chief (MC) a Spartan solider, basically a cyber-enhanced killing machine. The Cov. worship Halo. You explore Halo after crashing on it, finding soliders and killing Cov. as you go. While exploring you find that Halo is an ancient weapon created by the Forerunner, an extinct species. Inside Halo is a parasitic species that turns its food into zombies, this is the Flood. Halo was designed to kill the flood by starving it to death. To do this when Halo is activated it, along with the other Halos, fires a pulse that kills everything in a giant radius (25,000 light-years if I remember right). Naturally you don't want this to happen, so you rig the nuclear engines on the crashed Pillar to destroy the ring. End of game 1.
Game 2. A small number of Cov. showup by earth and get pretty much rocked by the humans. They didn't bring many ships because they had no idea that Earth is the human homeworld. You blowing up a Halo is pretty detrimental to the Cov. religion as they believe activating them is a way to heaven. There's a splitting amongst the Cov. as questions are raised. You get a chance to play as an Elite called The Arbiter. He's the general in charge of the force that should have prevented you from destroying the first Halo. So he's outcasted by the Cov. but secretly tapped by the guys in charge (Prophets) to destroy the rebelling sects of the Cov. Eventually the races of the Cov. split as the Prophets start going a little crazy, namely due to MC killing them. The Cov. try to activate another Halo but once again, you save the day. MC's storyline is pretty much the same as the first game. Kill Cov and Flood. But at the end you stowaway on the Cov ship that's headed for Earth. The Cov. are looking for a great religious item called the Ark, but you don't know what it is at this point. End of game 2.
I'll have to apologize for my recap of 2, mainly because I played through it, was pretty disappointed and went back to 1. So, I may have left out some things.
