First of all, this page contains SPOILERS. Do NOT read it unless you want the ending of Halo 3 spoiled for you. This is not appearing on the front page unless you've selected to view spoilers, and I've obscured the text by making it white on a white background; you'll have to highlight it to read. That's as much protection as I can give you.

As far as I can determine, this is the legitimate and true summary of Halo 3's plot; but I'll admit, I have not read it in its entirety myself, so if it's wrong, blame the author. It was posted by imKryptonite on the the360Forum.com; for what it's worth, he liked the game and felt it made up for Halo 2's shortcomings.

Here's an excerpt:

So, we start off with Cortana telling us why she picked John 117, and what he had that no one but her saw: Luck. We then witness our dear Spartan crashlanding into a jungle. Sergeant Major what's-his-name (the cool black dude everyone's so fond of) and his squad pick John up and bring him back to normal, at which point he also gets to see the Arbiter again, and find out that "they're with us now". You then fight yourself back to base, where Master Chief barely gets briefed before the whole place is overrun by Covenant forces. You get to shoot some guns and throw some 'nades and before you know it you've armed a bomb that's gonna blow the whole place to smithereens. Enter: Level 3, where you're going hog wild with the Warthog, racing down highways, beating stuff up, et cetera. You do a bunch of semi-important "oh and btw uhh new objective" stuff such as taking down an Anti Air cannon, while the Covenant is still digging happily into whatever the hell it is that's buried deep under Africa.

And then they light it. Now, the general theory is that, duh, that's The Ark, and it's here on Earth, and goddamn did the Prophet of Truth just fire all the goddamn rings in the entire universe from The Ark? But fuck us, we were all wrong. Turns out that wasn't really The Ark after all! Instead, it lighted up the sky, and brought upon us a motherfuckin black hole teleport portal, which some Covenant ships dive into to figure out just what the fuck is going on. However, there's a ship coming out of the portal as well ...

If you really want the rest, check out the forum thread.