It Followed Me Home
----------------------------------------------------------------------------------Objective: Find the crashed Flood ship
After seeing the Flood ship crash at the end of the last level, you have been tasked with destroying the ship and stopping the infestation. Head right at the start of the level, and work your way around the chasm towards the crash site.

You will come to a tight road. Follow it, and after it bends to the left, find the dead Brute on the left side of the path. Take his Gravity Hammer; you'll need it.

Continue down the path until you come to a group of marines behind sandbags. To the left will be the first group of Flood you have to fight. Use the Gravity Hammer, and collect the Bubble Shield near the sandbags.

Head into the building on the right of where you just fought the Flood. Inside, bash the enemy with your Gravity Hammer, then follow the Arbiter into the next room.
As you enter the next room you'll see a dead Brute with two Spikers laying next to him. Pick up the Spikers, switch weapons, and pick up the shotgun on the left of Brute, next to a dead marine. Now that you're properly equipped, move on out.

Move through the crates until you come to an area where marines are fighting the Flood. You'll see a yellow catwalk above the fray, so head up there to find a flame thrower.

Now, take out any remaining Flood and cross the catwalk, going through the door on the other side, and dropping down the hole in the floor.

Falling through the hole, you'll have to fight the Flood in very tight quarters. Use your flame thrower to take them out, but make sure you put as much distance between the Flood and yourself, otherwise you'll set yourself on fire as they run towards you.

Proceed towards the blue arrow and you will come out onto a catwalk. A cut-scene starts showing several Elites dropping out of the sky. The start of a new chapter!
----------------------------------------------------------------------------------Shadow of Intent

Continue following the blue arrow, passing the Elites fighting in the background. Follow the Elites into the building and through the hallway, destroying the Flood before you. As you follow the Elites into the second room of the structure, see that Cortana is on the crashed Flood ship, prompting a new objective.

Objective: Search the ship for Cortana.
Continue to follow the Elites and Arbiter towards the blue arrow marking the crash site. You'll come out onto a catwalk, where there is a mounted machine gun. Dismount the machine gun and use it to press the attack forward, destroying the Flood you come across.

Eventually, as you fight your way along the path, you will come to a point where you can no longer proceed forward. To your right you will see the crashed Flood ship. Jump down and pick up weapons right where you've dropped down.

Proceed up the ramp made by the ship's wreckage, and then jump into the bright yellow hole, taking you into the bowels of the ship, prompting a new chapter.
----------------------------------------------------------------------------------Infinite Devil Machine
Follow the tunnel of the ship. You will drop down another hole and then continue to follow the single pathway. The Gravemind will speak to you twice, and after the second time, you'll see the purple box where Cortana is supposed to be, which you then have to interact with Cortana's supposed location.

After interacting with Cortana's location, you will enter into a lengthy cut-scene that prompts the end of the level.