Holy craptacular. Welcome to the most direct level of Halo 3. This entire level is a backtrack of the previous level.

Anyways, as you already know...well...actually you don't technically know if you're really really really dense...so I won't
spoil it for you. What you DO know...is that a Covenant Cruiser crashed in the nearby city. It's what is INSIDE the ship which is the mystery.

It Followed me Home
Objective: Find the crashed Flood Ship

Ok...that objective gave it away...yes...there are flood in the ship...lots and lots of flood. But the flood are a lot more fair.
I'm not going to say 'easier' to kill...but more fair. You can now assassinate the smaller forms (hit them in the back) and they should die. And you can also shoot them in the gut with a Battle Rifle or Covenant Carbine for an instant kill. The Brute Shot is also an excellent weapon against the flood.

So work your way down the hill and all the way through until you reach the spot where you encountered the Brute Chieftain. To your left will be some Marines who are to be soon devoured by lots of Flood who jump on them from above. Yay! Wait...no yay. Kill the flood.

Once done, work your way into the building straight ahead. This room holds one of my favorite parts of the game, you can actually see at close range an actually Flood-spore infecting a Marine and turning him into a combat form (You'll see more of this in future levels). The next room holds some Marines fighting a bunch of flood and the rooms and hallways after that hold more flood.

But...you can find a nifty flamethrower in the first large room. As soon as you enter said room, you can see a Marine using a flamethrower from across the room...yes...he dies and there is nothing you can do about...I tried a few times...there was nothing I could do...*sad face*.

So grab the super-duper weapon and keep moving across the bridge and drop down. Proceed through killing everything in your path until...

Oh...and grab a Battle Rifle before you enter that hallway with the flamethrower...you'll need it soon.

Shadow of Intent

YAY! Backup! The Covenant Cruiser above launched some Elites to help you out. You should be able to find an energy sword somewhere down there. Head up the ramp with sword in hand, killing everything in your path. In the next area will be a hallway and a small room with lots of flood, dispose of them and head outside.

And then there are those spider like things which will change into flood that shoot at you.... yuck...use the Battle Rifle / Covenant Carbine or use sticky grenades. As you go outside, you can find a machine gun turret to your left.

Proceed ahead until you reach the crashed ship. Enter the crashed ship via the neatly placed ramp.

Infinite Devil Machine

Inside is a direct path until you reach a room with something in the center that you know you want to retrieve...yes...retrieve it...you fool...

Yay for you, you beat the level and survived an army of brainless flood that want nothing else but to eat your brains.