There is this group of aliens called the Covenant, right? And they discover human colonies and start blasting them away one by one. Then they reach the planet "Reach", Humankind's military outpost planet. In a desperate battle between the Covenant and the Humans, Reach falls. One ship, "The Pillar of Autumn", makes an unplotted jump into hyperspace followed by a Covenant few ships. The ship holds the last Spartan, John 117. Spartans are super soldiers the have been trained since age 5 to be the best of the best...

The POA comes across a ring world called Halo. The Coveys are hot on their tail and are about to take over the ship. Under the Cole Protocol (a very important document that talks about "you must not lead the Covenant to Earth, even if you must self destruct.") the ship's AI (Cortana) is taken off the ship by Master Chief, John 117.

MC finds out that Halo is a weapon with "vast, unimaginable power". He must now find the Control room of Halo to get to this weapon, so that he can destroy the Covey's before they destroy him with it. The Covenant believe that our "destruction is the will of the gods... ...and we are their instument".

So MC goes to find this Control room and Cortana starts flipping out and saying the Captain is in danger. Without being told what's happening, he's out looking for the captain.

After finding an officer's helmet camera, MC discovers that the "Flood" has been released. The Flood is like a virus that is very dangerous and reproduces very quickly and there is a lot of them.

Soon John is greated by 343 Guilty Spark (The Monitor).  He says that the "key" to the control room (which now MC thinks is used to kill the Flood) is in this building called the Library. MC wades through this area infested up to his waist with Flood, his shotgun his only companion. All types and forms of Flood are against him. Finally, MC retrieves the "Index" (the key) and heads back to the Control Room.

Upon arriving, Cortana gives him a very long lecture that the Monitor (which MC thought was his friend) is actually a bad guy.  Apparently, the weapon is designed to produce food for the Flood. That means everything in the galaxy, including MC, because the Flood consume everything. Since the monitor is an AI, he can't be hurt by the Flood.  Now the tables have turned. Master Chief's new objective is to destroy Halo.

Now the Monitor and his Sentinels (his "robots that do the dirty work") are the ultimate enemy. The monitor is determined to activate Halo's weapon, whereas MC is determined to stop him. MC destroys 3 pulse generators to stall Guilty Spark. After doing that, he now knows the Captain (Captain Keyes, of the POA) has been captured by the flood.

MC goes to rescue him but discovers the captain's been turned into a flood. Then he retrieves the data "inside Keyes" that will blow up the POA, causing a detonation that's enough to shatter the ring to destroy the Flood.

MC gets back to the POA, and the Monitor tries to stop him from blowing up the ship. But MC succeeds and Halo is destroyed. At the very end of the credits Guilty Spark is seen floating by, which means he has survived.

 

This is a no where near perfect recollection of the story, but it gives a bit more of a synopsis for people who have never played the game. If you'd like a better version of this story, please go out and buy Halo.  Now go to the real good stuff... HALO 2.