He starts shooting at you and you're forced to kill him. You come to one door and the door is locked but there is a decoder on it. Then, you hear a hissing noise behind you and you turn around, ready to blast anything that is there. But nothing is. You place your gun on your shoulder and press the button on the decoder. The door opens and a dead body falls into your arms.

You walk in, place the body gently on the ground and sweep the room with your machine gun. You notice a helmet on the ground. You take the recorder tape out of the helmet and watch the video. You first see the commander of the group assign one of the marines to lookout outside of the building. Then the marines discover a Covenant soldier lying on the ground, dead with a hole in the middle of its stomach. They all assume that it must've been friendly fire. They walk into a room and here a transmission from the soldier they left to keep a lookout.We've got contacts! Lots of 'em! Wha-They're not Covenant! They're emergin', tearin' through 'em, they're-auggghhh nooooooo! ". The commader orders Mandoza to go check what in the world happen, but he is interupted by a odd hissing noise. They here banging on a door then 100's of little creatures come crawling at them. On the floor, on the ceiling, infront of them, behind them..their everywhere! Then, the recorder goes black.

This brings fear into your heart, and suddenly you hear that same beeting on the door noise and see the same kind of little creatures. You try to kill as many as you can, but you hear beating on the door behind you. You start to panic as you turn around and look into the face of a creature....a creature so barbaric it isn't possible to describe. What is this creature? This creature is a human. A human that was turned into one of "them". Thats why that marine said "you wont turn me into one of those things." This isn't the way you want to die so you fight for your life. You rage on and escape out of the building after many detours. After reaching the surface, you find a group of marines. They were sent to find you because they lost your signal when you went into the building. More of those creatures start to attack you and your marines. After a while, flying robots come to your help. They are also trying to kill the creatures. You are all of a sudden transported at the top of the building and meet a round, floating robot. It says, "Greetings. I am the Monitor of installation 04. I am 343 Guilty Spark. Someone has released the Flood. My function is to prevent it from leaving this installation. But I require your assistance. Come. This way." He teleports you away and brings you into the "Library".

To activate the instalations defences, you must have a device to put into HALO's control panel known as the "Index". The Library is the building that contains the Index. After ten portals, many locked doors and three elevators, you reach the Index chamber. When you reach the floor that contains the index, the energy field shuts down. You retrieve the Index and Guilty Spark teleports you to the Control Room.

Guilty Spark gives the Index to you and you put it into the core of the control panel. Lights and holograms apear above the control panel but soon dissapear.

After you kill the sentinals that 343 Guilty Spark left to kill you, Cortana thinks of a way to destroy HALO! Cortana:We can't let the monitor activate Halo. We have to stop him. We have to destroy Halo. According to my analyses of the available data, I believe the best course of action is somewhat... risky. An explosion of sufficient size will help destabilize the ring, and will cut through a number of primary systems. We need to trigger a detonation on a large scale, however. A starship's fusion reactors going critical would do the job. I'm going to search what's left of the Covenant battlenet, and see if I can locate the Pillar of Autumn's crash site. If the ship's fusion reactors are still relatively intact, we can use them to destroy Halo.....We need to buy some time, in case the Monitor or his Sentinels find a way to activate Halo's final weapon without the index. The machinery in these canyons are Halo's primary firing mechanisms. They consist of three phase pulse generators that amplify Halo's signal, and allow it to fire deep into space. The power levels are enormous. I can't even begin to calculate the pulse's range. So, if we damage or destroy these generators, the monitor will need to repair them before Halo can be used. That should buy us some time. Cortana marks the location of the nearest pulse generator. She sets your shields to give off a EMP(Electric Magnetic Pulse) to damage the Generators. Before you reach the last generator, Cortana finds the location of the Pillar of Autumn's crash site. But, the only way to start the Self-destruct sequence is to have the Captain's authorization or his neural implants.(devices put into the captain so the Pillar of Autumn can detect that its him.) After destroying the last generator, Cortana uses 343 Guilty Spark's teleportion Grids to teleport you to the Covenant cruiser where the captain is being held.

Cortana:The Covenant network is in absolute chaos. From what I've been able to piece together, their leadership ordered all ships to abandon Halo when they found the Flood, but they were too late. The Flood overwhelmed this cruiser and disabled it. The Covenant are terrified the Flood will repair the ship and use it to escape from Halo. They've sent in a strike team to neutralize the Flood and repair the ship for immediate departure. I've got a good lock on captain Keyes' CNI transponder signal! He's alive! And the implants are intact! There's some interference from the cruiser's damaged reactor. I'll bring us in as close as I can. Cortana teleports you into the cruiser. You fall down, and Cortana soon realizes that the grid needs to be flipped upside down. You hit your head, shutting her up. Then you hear Keyes over the radio "Chief... Don't... Be a fool... Leave me..." You walk around the ship and soon come to a large whole. Cortana says that there are pools of coolant on the ground. She offers the advice to take another path because it is a very far fall. After being ambushed by waves of flood, she sugests jumping. The pools of coolant should break your fall. After you jump, you find the gravity lift for the ship and teleport up. While your searching for the captain on the ship, you hear him once again over the radio. "I... gave you an order, soldier! Now pull out!"He sounds like he's in pain. Minutes later you hear the captain over the radio screaming in pain. You and Cortana are getting very anxious. You enter the control room and see a large covenant creature standing there....with the captain's head poking out. They turned Keyes into one of the flood!