The story goes like this:
It is far into the future and hyperspace travel is a reality. The humans have ships exploring all over the galaxy. One day they encounter the Covenant, another advanced race capable of technology like hyperspace travel. Part of the Covenant religion seems to that all of humanity has to be wiped out, because once they discover that we exist, all they want to do is destroy us.

You are the last of the Spartan II's that were created, they are bio-engineered marines that have had a special robotic armor installed on them. All the rest were destroyed, but the ship you start on made it out with you (actually 3 others escaped but you don't find that out in the game). However, the Covenant followed your ship's hyperjump and got there before you. They've crippled your ship and are boarding in hoards, in the first level you must report to the captain then make your way through the ship to an escape pod.

One of the great things about Halo is how long the levels are. They usually take around an hour, except if your playing on a hard difficulty level. They can take hours because of how good the enemies A.I. is on difficulty levels like legendary.

The graphics are unparalleled, no other game has ever had such detail in such expansive landscapes. Physics in the game are amazing, if a grenade explodes around a dead body it will fly in the correct direction and possibly hit a wall and fall to the ground. All the marines and the enemies know what's going on. If a grenade falls next to the marines, they yell to duck or get down, then they scream if it explodes next to them. When the Covenant grunts see you they yell "He's here" or something like that, then they jump behind a box or wall to hide until you come closer. The enemies often wait until you reload your weapon, then they jump out and shoot you until you shoot them a few times.

The only thing I don't like is that there aren't bots in the multiplayer, otherwise the multiplayer is great and there are tons of options in it. I highly recommend getting this game, it's hours of fun and the multiplayer is absolutely awesome. So, remember these words: "If you own an Xbox, you own Halo."