When you begin the mission, you will find a large roadway with many Covenant forces. You are supposed to fight your way to a door that will lead you to a room with two turbine-like objects. Instead of going through this door, follow the road all the way to the end, killing all the Covenant on the way. Once you reach the end, there will be a Marine knocking on a door wanting to get in. If you listen closely, you can hear that it is actually an excerpt from Red vs. Blue where Simmons is talking to Griff trying to get into the base.