The Pillar of Autumn
Halo's story begins with Captain Jacob Keyes, commander of the UNSC ship Pillar of Autumn, on the bridge of his ship as it is emerging from a faster-than-light jump through slipspace. The ship has just fled the Covenant invasion of the colony known as Reach, and following the Cole Protocol procedure that requires making randomized jumps to conceal the location of Earth, has arrived at a large ringworld structure situated at a LaGrange point between a large gas giant and its moon.

Keyes inquires with the onboard AI, named Cortana, who appears as a foot-high purple hologram of a young woman, about whether or not they have shaken their Covenant pursuers: it appears they have not. Keyes orders the crew to prepare for combat.

Crewmen elsewhere on the ship reviving John 117, the Master Chief, a SPARTAN II soldier wearing Mjolnir Mark V power armor, from a cryo chamber. After that, Covenant Elites break into the cryo control room, killing the crewmen, and you are instructed to go to the Bridge to meet with Captain Keyes. The crewman accompanying you is killed by an explosion, but between now and when you reach the Bridge for a second cutscene, you are largely safe from harm. You are weaponless, and while you walk past and through several battles between Grunts, Elites, and UNSC Marines, getting killed is hard if not impossible, and invisible walls separate you from most hostile units, so you can't get in much trouble.

When you arrive on the Bridge, Keyes gives you your first orders: you are to take the onboard AI, Cortana, with you and escape to the surface of the ringworld to prevent her from being destroyed or captured by the enemy. Keyes gives you an empty pistol.

You spend the rest of the level assisting Marines in repelling the Covenant boarders, that have attached their boarding craft to the docking points for the Pillar's escape capsules. After a brief detour through parts of the ship's maintenance accessways, you reach the last available escape capsule with a handful of Marines, and the pod launches towards the surface of the ringworld. On the way down, Cortana notes that Keyes is still onboard the Pillar of Autumn, and piloting the ship down on manual control.

Editor's Note: There is a lot of background material about Reach, the genesis of the SPARTAN program and its creator, Dr. Halsey, as well as the other SPARTANs and the special mission for which the Master Chief, Captain Keyes, Cortana and the Pillar of Autumn were originally chosen in the novelization The Fall of Reach by Eric Nylund. While informative, it is not, strictly speaking, essential to understanding the plot of the game although it does lead to a greater appreciation for the detail that Bungie has put into the story. However, those events and details are outside the scope of this summary.

Halo
The second is the game's titular level, even though none of the characters in the game have even mentioned the word "Halo", which we find out later is the name the Covenant use for the ringworld structure.

The escape capsule's air brakes fail and it crashlands, hard, on a grassy knoll near a waterfall. You awake to find the pilot and other Marines aboard the capsule dead, and a convenient pile of weapons, ammunition, and grenades just outside. As you begin to explore the area around the capsule, a Covenant dropship will arrive and drop off troops apparently looking for you. Later, two Banshee flyers will attack.

Your mission on this level, broadly speaking, is to link up with other Marine survivors. On the other side of a bridge and on the other side of a rise away from the waterfall area where your pod crashed is a structure where a group of Marines are attempting to hold off waves of Covenant units (Elites, Grunts, and Jackals) brought in by dropships. When all are defeated (whether or not you save any Marines) a Pelican dropship piloted by Foe Hammer will arrive, drop off a Warthog jeep, and take away any excess survivors. It will be useful to try and keep at least one or two Marines with you in the Warthog, especially on the LAAG cannon mounted on the back. Marines cannot drive the Warthog, as they do in Halo 2 (and some might say that is a good thing). The level is divided in half by a subterranean system of tunnels bifurcated by a chasm that you can cross by activating a light bridge guarded by other Covenant units. On the other side of the bridge, you emerge from the tunnels in another area with hills, rockslides, a river and a waterfall. Here you must link up with Marines from three crashed escape pods; after each of the three encounters are finished, Foe Hammer will arrive again to take survivors aboard, and after the last one, you will go with her as Cortana has revealed that Captain Keyes has been captured by the Covenant.

Editor's Note: This is one of the few non-linear levels in Halo, as each of the three escape pod locations is accessible, and can be played in any order. Some players have manipulated the circumstances of these scripted encounters to create "megabattles" by concentrating reinforcements in one location. It is also possible to collect multiple Warthog jeeps on this level, as if you leave the one you took to get to an encounter in the central "hub" area, Foe Hammer will bring another one to you when she airlifts Marine survivors.

Truth and Reconciliation
The Covenant cruiser Truth & Reconciliation was disabled by Cortana before the Pillar of Autumn crashed. Captain Keyes, as well as other Marines, are now being held on board, and your mission is to find a way into the cruiser, rescue the Captain, and then escape.

The approach to the cruiser happens on a cliffside after nightfall, and most of the action is based around stealth and sniping. At the entrance to the cruiser itself, a device called a gravity lift, we see Hunters for the first time in the game: massive, tall, blue armor-plated beasts with FRGs (fuel rod guns) molded into their right arms and huge metallic shields on their left.

From then on it is a corridor crawl inside the Covenant ship. You'll pass through several large areas, including a hangar deck and a control room, before reaching the second of two brigs where Captain Keyes and his men are held prisoner.

Normally in Halo you don't need to worry about saving Marines; but in order to complete the level you have to safely escort Keyes back to the hangar bay to escape in a dropship. As Keyes often leaps ahead of you armed with a Needler, this is sometimes difficult.

In the brig scene, Keyes lets you in on a few things: that the Covenant call the ringworld "Halo", that it is supposedly some kind of superweapon that will give whomever controls it the ability to control the fate of the universe, and that they are seeking its Control Room.

Silent Cartographer
You accompany a group of Marines in two dropships to an island where Cortana tells you there is an installation known as the silent cartographer, which is a map room that should tell her the location of the Control Room. You need to clear the beach, clear out one minor installation of Covenant to deactivate a security lockout so you can infiltrate the second installation, a deep shaft at the bottom of which is the map room. When you activate the map, Cortana tells you that it shows her where the Control Room is, and that the structure appears to be a "shrine or temple" of some sort, which she finds odd. Foe Hammer arrives and takes you off, whereupon Cortana opens a huge hatch on the top of the island and directs the Pelican to carry them into it, towards their next destination, the Control Room.

Assault on the Control Room
Easily one of the game's largest levels, Assault on the Control Room features a broad mix of indoor and snowdrifted exterior environments, as well as a mix of play using vehicles, including a Warthog jeep, the human Scorpion tank, and Covenant Ghosts, Banshee flyers and Wraith tanks. Unlike in Halo 2, some vehicles, such as the Wraith, cannot be driven, even if empty; and killing Covenant pilots nearly always destroys the vehicle, whereas human vehicles like the Warthog are utterly indestructible. The objective is fairly simple: you follow arrows on the floor towards the Control Room at the end of the game; a large pyramid structure, inside of which is a huge chamber showing a holographic map both of the planetary system and the surface of Halo itself.

Once you get there, you connect Cortana to the system. She discovers that Halo was built by something called a Forerunner; they called it a Fortress world. She also finds that Captain Keyes, who thinks he is looking for a hidden weapons cache elsewhere on the ring, is about to make a potentially fatal mistake; she indicates that the Covenant themselves do not seem to be aware of the potential danger. She orders you to find Keyes and stop him.

343 Guilty Spark
Foe Hammer drops you off in a swamp exterior near Keyes' crashed dropship. You proceed through the jungle, past minor Covenant resistance (Grunts and Jackals only, no Elites on this level at all) into an underground structure, where everywhere there is evidence of chaos among the Covenant ranks.

On the way you'll find a marine that will explain a bit of what happened-- that something came and took the other Marines, and that he escaped by playing dead. He calls them only "monsters", and if you stand in front of you too long he'll fire his pistol at you.

In the middle of the level you'll reach a room with only one exit and entrance and a cutscene will play; you see the recording of a Private Jenkins, as he, Sargeant Johnson and Captain Keyes and his men land, proceed into the installation, find a set of doors locked by the Covenant, and find themselves attacked by Flood infection forms; sporelike parasites that infect Human and Covenant units and transform them into gurgling Flood combat forms.

After the recording you'll be attacked yourself and you'll be forced to flee back to the jungle surface, pursued by Flood and occasionally watching battles between either Marines and Flood or Covenant and Flood. When you finally make your way to the surface, another party of Marines is waiting there, and Foe Hammer contacts you by radio to instruct you and the troops to head for a nearby tower structure for pickup. Along the way the Flood harass you from all sides.

When you reach the tower, the final cutscene plays, where the spherical, blue, humming, Marathon-symbol-wearing 343 Guilty Spark, the Monitor of Installation 04, appears before you and explains that the outbreak of the Flood must be stopped, and teleports you to the Library to retrieve the Index and activate Halo to "contain the Flood".

Editor's Note: This level is a real gem; a fine balance of story, environment, and gameplay. It doesn't feature a lot of combat, aside from the second half with the Flood. You get to fight Flood with Marines at the end; sadly, this is the last time you'll see a living Marine in the game. There aren't many Covenant units at all. But the atmosphere of the level, the ambient music and sounds, the jungle rain and fog, are all just perfect.

The Library
Abandon all hope, ye who enter here... but at least pay attention. In this level you must ascend to the top of this massive, dark structure and retrieve the green glowing artifact called The Index. With the help of 343 Guilty Spark and his floating Sentinels, armed with beam weapons, you fight off the Flood that are infesting the structure. The Library is one of the most reviled levels; all the corridors are dark and each of the "levels" you proceed through look the same. You only fight Flood infection, combat and carrier forms here: no Marines or Covenant to break the monotony and no vehicles, or outdoor areas: not even a window.

The level is worth playing to hear all the things 343 Guilty Spark says, if you can hear them over the Flood gurgles and the explosions of grenades and rockets. Apparently, Halo was designed and built to contain and study the Flood. He calls you "reclaimer", leading to rampant speculation that either humanity themselves are the forerunners, or that the Master Chief goes back in time later in the Halo series. He chides you for your choice of weak weapons, urges you to get a better suit, and indicates that soon the Flood will begin to alter the ring's atmosphere.

When you finally reach the Index, Guilty Spark takes it from you, saying that your biological nature makes you vulnerable to the Flood, which requires him to keep the Index safe for transport. Then both of you teleport to the Control Room.

Two Betrayals

While returning to the Control Room with 343 Guilty Spark, the monitor is mentioning that factors in being susceptible Flood infection are biomass and intelligence.

The monitor gives the Index back to you to insert into the Control Room's panels, and the system appears to start powering up; at that point, a red-eyed "rampant Cortana" appears to stop the process. She reveals that activating Halo doesn't kill the Flood, but rather all sentient life, and 343 Guilty Spark confirms it. Cortana takes possession of the Index; and when the Master Chief refuses Spark's order to return it and evacuate Cortana from the Core, the monitor summons sentinels who float up from the shaft underneath the holographic map.

Despite advanced Forerunner technology, the Sentinels aren't nearly as difficult opponents as what you'll face further along, and the game has helpfully started you out with a plasma pistol and it's tracking alt-fire bursts, perfect for downing Sentinels in a single shot. Hit them when they are near to one another and the explosion can damage other nearby targets, making it even easier. The transparent walls around the control room's center ring make handy cover.

When the Sentinels are dealt with, Cortana reveals your new ultimate mission: to destroy Halo to prevent its activation. Ultimately, to do so you'll use the fusion engines of the Pillar of Autumn. Before that, however, she'll ask you to find and disable three pulse generators located in the canyons around the Control Room. Coincidentally, these are located in the same canyons you passed through on your way to the control room in the first place; nearly all the level geometry from Assault on the Control Room is reused here, and the pulse generator rooms are actually present in that previous level, just behind locked doors. You won't go as far as the initial indoor areas and the landing spot in AotCR, although it is possible to get there by using some vehicle tricks to push yourself through locked doors.

Along your route you'll fight Flood as well as Covenant units, while the two groups are themselves locked in combat. Where Banshees were an unexpected bonus in AotCR if you played a few tricks to get them, here they are essential, as each of the entrances to the pulse generators are above ground and accessible only by air.

Although the geometry is the same, the mood is entirely different; the darkened sky, the falling snow, the music and the complete lack of any live human marines all create a sense of foreboding. For sheer atmosphere, this is one of the best levels. Watching Flood and Covenant at each other's throats in the dim light, you really feel like you are completely lost and alone.

The first outdoor area from AotCR is as far as you'll have to go; that's where you'll pick up the last Banshee you need and head back, towards the last pulse generator.

Bungie's played a nice little trick here; at the second pulse generator, there's heavy Flood presence. Why is a bit of a mystery-- not to mention how they got up there! Given that you're there trying to deactivate Halo, something you'd assume would be in the Flood's best interests, it's mysterious that they seem to be trying to stop you. Then again, perhaps their much-vaunted intelligence doesn't really count for much.

Once you've taken out the pulse generator by stepping into it, Flood leap from the side tunnels and enter through the door behind you. On legendary, getting out of this room alive takes a bit of doing, as the Flood entering the room are armed with rockets.

The last generator, though, is undefended, and the eerie quiet in the room once you've entered is clearly designed to make you wonder what's going to happen when you disable it.

All that does happen is that Cortana says she's found a way to access the ring's teleportation grid and power it with your suit, something she says she's only willing to try once, and whisks you into the next level. To destroy the ship, she says, you need either Captain Keyes or his neural implants; so the next task is to find him.

Keyes

Given that you've just both betrayed and been betrayed by the floating, glowing Monitor of Installation 04, determined yourself to destroy the fantastical floating ring in space called Halo that just happens to be a life-killing superweapon, and in the process eliminating not only the Covenant, most of your surviving fellow humans, and the Flood now loose on the ring, and are about to embark on a mission that ends with you ripping the goo-covered neural implants from the skull cavity of your now grossly overweight and Flood-infected former captain, it's not out of place that Bungie should try to lighten the mood with a bit of comic relief. Which is exactly what they do as Cortana teleports you into a damaged Covenant cruiser upside-down and then apologizes for it.

Frustratingly, you've been teleported to a hallway just next to your eventual destination. You can peer through the perforated wall into the Chamber where the Keyes blob is, but without some precision trickery involving a sunroof and a Covenant shield generator, you're not going to get there except by going around the long way.

The captain knows you're there as well, and is still human enough to warn you against attempting a rescue... again. Bungie's not fooling anybody here, though. From the agonizing sound of the captain's voice, he's not making it out of here alive.

Not as much story is revealed through the gameplay of this level than in preceeding levels. We've already seen Flood and Covenant fighting, and you'll get a lot of this. At most points, unless you really feel like jumping in like Rambo without a jock strap, it's best to let the sides even up on each other before taking out the stragglers.

Early on you'll be forced to leave the ship and reenter it through a gravlift, so you'll certainly get some more deja vu, as you did in the previous level.

This level is also home to one of the nastier tricks, where large number of Flood will spawn in a cavity above an open ceiling panel and jump down behind and in front of you. The effect is rather like every monster closet in the Doom series; you've got to wonder what the Flood are doing up there and why, in discrete groups, they descend upon you exactly as you pass through a very specific point on the floor. Of course, most scripted encounters are in principle very similar; the tight quarters just emphasizes the mechanics of what is happening.

One location you'll recognize on your way through to the captain is the hangar bay. This may, or may not, be the same ship you were on earlier in the game, the Truth and Reconciliation. The game doesn't tell you, although author of Halo: The Flood, William C. Deitz, says that it is. No matter; all the polygons are the same, only the cast and lighting have changed. Mark the spot well; just as in that earlier level you have to return here to escape, this time in a stolen Banshee instead of a dropship. You've also seen your last dropship, as Halo 2's phantoms take over that role in the next game.

In the cruiser's control room you find what's become of the captain: a bloated flood-filled bag of pus growing tentacles (one of which is smoking the captain's pipe) and with a barely-recognizable face. Which you then punch to retrieve the neural implants before fighting your way back to the hangar.

Once there, getting out of the level while avoiding a fight (should you choose to) is about as easy as it was in Truth and Reconciliation; as the Banshees enter the hangar, you can drop down on top of one without dying. Enter the vehicle quickly and make your way out the open bay door and you're home free.

The Maw

Of course, you might be home free, but you can never go home. That's what your return to the Pillar of Autumn seems to prove. Although this level does include areas you've seen before, like the bridge, and the cryo chamber; of all the reused levels less of this architecture is actually reused. A large portion of the level is taken up by the engineering room and the warthog racetrack that leads to the longsword in the hangar bay.

But before all that you've got to get to the bridge. Flood have taken over the ship, as 343 Guilty Spark warned you back in The Library, but the Covenant haven't given up without a fight, as they are also trying to use the PoA to escape from Halo. A few sentinels will harass you from outside the ship, but that's really just for window dressing, unless you plan on using one of them to surf down to the bottom of the level.

Once you make your way to the Covenant-held bridge, Cortana will set the ship for self destruct, only to be stymied by 343 Guilty Spark, who is catching up on human history, what he calls "all of our lost time" from Engineering. The Master Chief's answer to this conundrum is to detonate the engines manually, with a rocket or grenade to each of the six engine casings.

After that, it's a quick elevator trip to pick up a ride and head to the launch bay, where the Master Chief, Cortana onboard, get on a Longsword fighter and escape from the Autumn, which then self-destructs, destroying itself and Halo in the process.

Echo 419's pilot, Foe Hammer, was supposed to meet you at an external access junction along the way, but was shot down by Banshees in the attempt.

On board the Longsword, Cortana says she detects nothing but Dustin Echoes dust and echoes, implying that no one else has survived the cataclysmic destruction of Halo, but that the elimination of the Covenant fleet and the prevention of the escape of the Flood from Halo justifies this.

After the final credits roll, we see that at least one other did survive, as 343 Guilty Sparks streaks by the screen through empty space.