Upon completing the level on the Normal, Heroic, or Legendary difficulties, the player will earn the "Holdout" achievement and 20 Gamerscore. Upon earning 15,000 points in Campaign Scoring, the player will be awarded with the Demon achievement and 10 Gamerscore.
The level begins with the Master Chief, the Arbiter Thel 'Vadam, and Sergeant Johnson landing in an underground UNSC base. When they disembark, several Marines are amazed to see a Spartan II. As they walk through hallways littered with wounded UNSC soldiers, Commander Keyes informs the Chief and Johnson that Truth has almost destroyed the UNSC Home Fleet, and only a few ships are left. They soon reach the command center and speak with Lord Hood, discussing a plan for an all out attack on Truth's army.
Unfortunately, the Covenant then jam the Human communications network along with the rest of the base, shutting off the power. Truth appears on the main view screen, telling the Humans that their world will burn. The base is then attacked by a large contingent of Covenant troops. The Master Chief first clears the south hangar, allowing the UNSC personnel to evacuate. Unable to fight off the invaders, Keyes orders a bomb placed in the operations center. Meanwhile, Jiralhanae capture the barracks and take many Marines prisoner. The Chief and the Arbiter retake the barracks and rescue as many Marines as they can. They then bring the survivors up to the landing pad where Keyes is waiting with a Pelican. After fighting off another group of Jiralhanae, they meet up with Johnson who informs them that Jiralhanae have seized the Ops Center and disarmed the bomb. Keyes orders the Chief to retake the Ops Centers while the others evacuate. The Chief clears out the Covenant inside the Ops Center and rearms the bomb. He then makes his way back to the south hangar, where he finds a service elevator. Moments later, the bomb detonates, sending the elevator into free fall and causing the Chief to black out.
Kilo 023 touches down. Medics load an injured marine on a stretcher and move. The Master Chief, Thel 'Vadam and Johnson get off. Marines sitting by the landing pad see the Chief get out of the Pelican.
Commander Miranda Keyes comes out onto the Landing Pad to meet Johnson, 'Vadam and the Master Chief.
They salute, as she salutes back and smiles.
Keyes shakes the Chief's hand.
Down the hallways of the base, wounded Marines lay against the walls. Two Marines chatting to each other notice Commander Keyes and the Master Chief. They make way and salute.
As 'Vadam walks by, the Marines lower their hands and stare in astonishment.
Medics race by with an injured Marine on a stretcher. Johnson tends to an injured Marine.
Lord Hood appears on the main screen.
The power cuts completely.
The Prophet of Truth suddenly appears on all the screens.
The power returns.
Commander Keyes thinks, and realizes Truth's meaning.
Miranda walks up the stairs and draws her pistol. She pulls the slide back.
She lets go and moves on.
Know Your Role...
(Co-op Change) Sergeant Johnson: "Chief, move out. You too, Arbiter. The Commander will brief you as you go."
The Master Chief meets with two Marines from downstairs.
(With IWHBYD Skull) Marine: "This base was built for some 20th century war, and it's pretty damn cool!"
The Chief follows the Marines downstairs.
The Chief finds many weapons to stock up on.
The Marine opens the door. The Chief walks on through to find Marines in a large cave setting up defenses for the Ops Center.
The cave shakes.
(Co-op Change) Gunnery Sergeant (Reynolds or Stacker): "Agreed. Chief, Arbiter. Get there."
The Master Chief proceeds on, meeting up with two Marines.
The Chief grabs some Fragmentation Grenades from a weapons case near the door, then opens it, and lets the Marines lead the way.
The Marines find the vehicle access. A Warthog is in battle with many Covenant soldiers. Unggoy lob plasma grenades and destroy it. The Chief and accompanying Marines engage and defeat the Covenant. A Jiralhanae wielding a brute shot enters the vehicle access with a group of Unggoy. The team eliminate the Covenant troops and push onward. They go through the open door and defeat the Covenant there, which consists of many Unggoy and Kig-Yar Snipers. Proceeding, the Chief and Marines reach the hangar.
The Chief and Marines go on the offense and secure the deck. Multiple Phantom dropships enter the hangar, delivering groups of infantry in an effort to retake the stategically important room. From a distance, Hornets and Banshees are seen engaged in an aerial fight near a CCS-class battlecruiser which keeps sending Covenant reinforcements to the base. The Chief and Marines make use of several turrets and defend the hangar. The UNSC forces eventually prevail and the Phantoms retreat.
The Pelican hanging on a crane in one of the docks slides out and is released. The dropship lands to evac the Marines in the hangar, while the Master Chief restocks on weapons and ammo.
If the player is in the way of where the Pelican lands:
If the player lingers:
The Chief heads back to the Ops Center. Marines pass by for evac. Back in the vehicle access, two Marines, staying behind until the next evac, hear a loud banging as well as the sound of insectoid wings beating in the ventilation pipes as something makes its way through.
The Chief heads back into the corridors, with the two Marines following.
The COM systems remain silent. The banging and fluttering noises continue on.
The Master Chief meets with the Marines in the cave, defending the Ops Center.
A single Yanme'e suddenly bursts out of the vent and picks him up. (The Yanme'e can be killed before it picks up the Marine. If it does, he can still be saved by simply shooting the Yanme'e, which will drop the Marine).
A massive swarm of Yanme'e starts pouring out the ventilation shafts, and engulfs the room.
The Marines open fire on the swarm. The Chief finds a turret and unloads on them. The swarm eventually falls. Several stragglers scurry about on the ground. The Chief easily dispatches them.
Marines in the Ops Center below open the doors, letting the Chief in.
Gift With Purchase
Butchers in the barracks.
The Master Chief meets with Johnson smoking a cigar and other defending Marines setting up an explosive device, but Thel 'Vadam isn't with them. (Single Player Only)
Johnson drops his cigar and stamps on it.
The feed disconnects.
(Co-op Change) Johnson: "Chief, Arbiter. Follow me."
Johnson leads the Chief to the back of the room, guarded by four Marines.
The Master Chief takes weapons and ammo from a weapons locker.
If the player stays near the bomb:
The Jiralhanae disperse, intent on tracking down the source. The Chief surprises them and manages to take several Jiralhanae out. Reinforcements soon enter from an open door near the Chieftain, many wielding Covenant carbines. Once most of his pack have been slain, the Chieftain brandishes his hammer and charges at the Chief, usually engaging an invincibility unit. Regardless, the Chief prevails and completely ousts the Jiralhanae pack.
The Master Chief continues on through a corridor, but it's caved in. Finding a hole in the floor, he jumps down into a shaft. At the bottom are several pipe entrances, which Yanme'e fly in and out of.
Master Chief jumps down through vertical shafts into a canal, where he finds Thel 'Vadam, fighting off a small group of Yanme'e. (Single Player Only)
After 'Vadam takes care of the Yanme'e, he and the Chief regroup.
The Master Chief and 'Vadam enter the barracks. They see a captive Marine thrown hard into a pillar, where multiple Marine corpses lie. The Marine dies and falls limply to the ground, and the Chief and 'Vadam rush in. One Marine being held by a Jiralhanae begs for mercy as it decides to kill him too.
The Chief can opt to attack the Jiralhanae, which releases the Marine prisoner.
If the player fails to save him, the Jiralhanae breaks his spine and throws him on the ground behind him, laughing.
If the player saves the Marine:
Further down the corridor a Jiralhanae Captain holds a Marine by the ankle. Another marine is being held by the neck. Their survival depends on how fast the Chief is at attacking their captor(s). (On higher difficulties, the Jiralhanae are reinforced by Kig-yar infantry).
The Chief and 'Vadam kill the Jiralhanae and save the prisoners (optional). While the freed Marines rush for weapons, the Chief and 'Vadam move through the barracks, killing more Jiralhanae and freeing another group of prisoners.
After taking out the Brutes, the surviving Marines grab weapons from more piles of corpses, taking a second to grieve for their comrades while the Chief and 'Vadam resupply and reload. They continue their rescue throughout the barracks, defeating the remaining Jiralhanae, including a Jiralhanae War Chieftain with a detached plasma turret, and rescuing (a maximum of) seven Marines.
Once the barracks are cleared:
(Co-op Change) Gunnery Sergeant (Reynolds or Stacker): "You did the best you could, sir. Let's get the survivors up to the hangar. There's an elevator just outside the barracks."
The Chief leads the survivors outside to the lift. He activates it and they wait for it to come down. While they wait, matters get worse.
The elevator arrives. The Chief, 'Vadam and the Marines get onboard and take it up to the flight deck. While the team heads up, Kilo 23 gets into trouble.
Kilo 23 takes off, getting out of the Jiralhanae's reach. Noticing the Chief and 'Vadam, the Jiralhanae Jumpers attack them. Reinforcements enter from above the observation room as well as from another door. The Chief, along with his allies ultimately defeat the Jiralhanae and secure the flight deck, a door at the road below opens. Johnson arrives, holding an SMG and leading a group of surviving Marines. The squad is under fire from a group of Yanme'e.
The Chief and 'Vadam assist Johnson and his squad in killing the aliens. The few Yanme'e are quickly eliminated. Kilo 23 returns for final evac. The barracks survivors and Johnson's squad board the Pelican dropship.
Johnson and Thel 'Vadam board the Pelican, and it takes off. The Master Chief goes along the road Johnson previously came from, passing by destroyed Warthogs and dead Marines. (In Co-op, 'Vadam will accompany the Chief.)
Last One Out, Get the Lights
Bomb armed. Time to go.
The Chief reaches a door at the end of the road.
The Chief walks on through to another roadway, with Covenant plasma cannons set up. Using the Plasma Cannons and/or using all available cover, he clears out a large contingent of Unggoy and Kig-Yar along with a pair of Kig-Yar Snipers. The Chief soon gets back to the Motor Pool next door to the Ops Center.
If the player is taking a while:
If the player still stalls:
The Chief heads back up the corridor behind the Ops Center, and sneaks in. Inside, he witnesses a Jiralhanae War Chieftain communicating with the Prophet of Truth via the Ops Center communications. With the Chieftain are a several Unggoy and a pair of Jiralhanae Captains. (On higher difficulties, another Jiralhanae mans a plasma cannon at the far end of the room.)
If the Chief isn't spotted, he can spy on the conversation between Truth and the Chieftain.
Truth disconnects from the screen.
As the Unggoy and Jiralhanae Captains get busy, the Chief takes them by surprise and defeats them. The War Chieftain opens fire on the Spartan with a fuel rod gun but the Chief manages to overcome him, taking back the Ops Center and the bomb.
If the player waits around a bit:
The Master Chief reactivates the bomb and the alarm sounds.
The Chief heads back into the cave, where Unggoy run wildly. He then heads through the corridors.
If the Chief stalls:
The Chief continues back out to the vehicle access, witnessing a large swarm of Yanme'e fleeing. Making his way through several panicking Unggoy, the Chief heads back down the corridors to the hangar.
The Chief gets to the hangar, where Phantoms flee, abandoning many Unggoy and Kig-Yar to their fates. The doomed Covenant run around in panic, some trying to catch the fleeing Phantoms.
If the player takes their time:
If the player continues to linger around:
The Master Chief makes it to the service elevator, and activates it. The door doesn't fully shut. Something explodes and flames flicker through the gap.
The elevator descends. The Master Chief hears a massive boom. He looks up and sees the flames roar through the open door. The violent shaking disconnects the elevator and sends it plummeting down. It crashes and the Chief blacks out.
The screen goes black.