Major Halo 3 Spoilers.
[spoiler]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 ...
Yes. The fucking flood, they're back again, and goddamn if you're not gonna backtrack through the area you just finished to fight some more flood. I'm being completely honest when I say that they're not nearly as annoying as before. Could be that you now actually have people on your side as well, could be that they're better balanced, could be lots of things. Anyway, you eventually board the godawful ship the crashed into your beautiful planet, and lo and behold, there's a message from Cortana aboard.
Basically, she's saying that the solution to all this is through that portal, and that we must risk everything, blabla. Seing as Master Chief is one charismatic bastard, we go through, and my oh my, end up at some distant place in space, at another Forerunner built structure: No, not another Halo, but The Ark. It's kinda similar to the Halos, in that it's created by the Forerunners, but has a different shape I'm not quite sure how to describe. You crashland and fight some covenant shit, revisit places that remind you alot of Halo (you need to get a cortagropher, for example) and pull some wires to get inside some important places, and you even team up with the flood while you're rushing the Prophet of Truth (which is just chillaxin here at The Ark), which the Arbiter eventually kills. So yay, right? No, not yay, not yay at all actually, because Gravemind and the Flood just turned evil again, and goddammit you're so tired of this shit, and then another, unfinished Halo replacement ring (yes, yes, yes) rises up from the ocean of The Ark. So you're thinking "oh shit lets get out of here Gravemind is here remember" but then you're all like "oh shit Cortana I miss her" and you go on a journey deep within this hellhole to find Cortana, and you do. So you get the fuck off the Ark, get on the Ring, meet up with 343 Guilty Spark at one point as well, and you're gonna light the fucker. As you guys remember, you need all 7 rings to blow up the universe, while a single ring will just blow up shit that's in its immidiate vicinity. So you board the goddamn ring (also forgot to mention, but that female captain you work closely with? Yeah, Prophet of Truth killed her), and the Flood is dropping pods on your ass like nobody's business, but you meet up with that black Sergeant Major (Johnson, is that his name? I'm real bad at names, sorry) and you're all set to activate the ring when 343 proclaims the ring to be ready in "a couple of days". After realising that you don't actually have a couple of days, shit goes bad and 343 tries to kill you all. However, you are the hero and therefore you win. However, activating the ring prematurely will result in not only the destruction of everything that's close by, but also, tadada, The Ark, for some reason. This however is just dandy, seing as Gravemind is still on the goddamn Ark.
So you activate it, and run like fuck. PS: Johnson died. And so, the best fucking finale they could have ever pulled off: The last section is a getaway on a warthog, just like in Halo. And goddamn, it's glorious. There's no timer (except Cortana frantically yelling "go go go go!" all the time), and even some checkpoints en route. So you barely make it, land in the ship Johnson left behind, Arbiter gets in the driving seat and speeds off towards the portal.
So, cut to memorial on Earth, where it's revealed that, yes, Master Chief is in fact dead. However, The Arbiter made it, which seems kinda funky, right? He also says something along the lines of "were it only so easy" when the General proclaims that The Arbiter was with him when he died.
Credits.
But wait, just like in this post, there's more after the credits! Turns out the Portal got blown up as well, and only half the ship made it through (ie Arbiter making it to earth, as he was in the driving seat up front, while Cortana and MC were chillin in the back), and yay, MC is still alive. Cortana drops a beacon but says it might be years before anyone picks it up. MC however, says "fuk dat" and goes into hypersleep in a pod like the one you woke up from in Halo, with his final words being someting along the lines of "Call on me when you need me". I forgot to mention, but along the way you fight several of those badass walkers you took down one of in Halo 2, visit areas almost identical to areas in Halo, participate in some huge badass battles, and generally having a good time.
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There, the entire plot of Halo3 spoiled in one, giant badly-written post. I hope you feel good now!
Source: http://forums.somethingawful.com/showthread.php?threadid=2627690&pagenumber=2#post333465145
