A Brief History of Halo
For those of you left who haven't played Halo, Stuffo has included a Cliff Notes version of the story so far. This will set the stage for Halo 2
The year is 2552. Man has colonized several planets out of our solar system. Humanity's thriving but fragile efforts to expand into space go unchallenged until a strange, alien radio transmission in broken English promises our destruction.
A group of alien races bound together by religion start to destroy planets colonized by humans. They call themselves The Covenant, and they claim the destruction of mankind has been ordained by their gods. Their ultimate goal is Earth, but first they must find it.
A war rages for years between the Covenant and mankind. We lose on every front. The Covenant Juggernaut slaughters millions of humans. In an effort to tilt the odds in our favor, we create the Spartan program to train an elite cyborg soldier unit. The Spartans soldiers wear a special type of armor created with stolen Covenant technology. The Spartans are powerful but are unfortunately all destroyed by the Covenant in one fell swoop at the Battle of Reach. All but one.
Enter the Chief
Master Chief, the only surviving Spartan is on board the naval cruiser Pillar of Autumn (PoA) as it flees the defeat at planet Reach. During its escape, the PoA comes under Covenant attack. In an effort to lure the Covenant away from Earth, the PoA makes a blind jump into light speed. When the PoA drops out of light speed, the crew finds themselves near a huge ring-like structure in orbit around an unknown planet. They also find the Covenant waiting for them.
The ship's captain orders the evacuation of the PoA as it comes under attack from Covenant boarding parties. Captain Keyes orders the Master Chief to take the PoA's shipboard A.I., Cortana, and escape so that Cortana does not fall into enemy hands.
Repelling boarders along the way, Master Chief escapes the crippled PoA on the last lifeboat. He and scores of other fleeing Marines and Naval personal head for the strange ring world as Captain Keyes crash lands the PoA. Hot in pursuit, the Covenant sends assault forces to the ring world to mop up the human survivors.
This is the central conflict of "Halo." As the stranded Marines regroup and organize, they wage guerilla war against the Covenant. The longer the Chief and the Marines spend on the Halo, the more they begin to unlock its secret.
What is the Halo?
After rescuing the captured Captain Keyes from the Covenant battleship, the Truth and Reconciliation, Master Chief learns that the Covenant believe the Halo holds a powerful weapon. It turns out the Covenant intends to seize control of the weapon and use it against mankind.
Now the Marines struggle to get the weapon before the Covenant does. After many fierce battles to control the Halo's map room and control room, Captain Keyes takes a Marine unit with him to retrieve Halo's mysterious weapons cache. Cortana is installed in Halo's system and learns the true nature of this weapon. She sends Master Chief to stop Keyes and the Covenant from unlocking this terrible weapon. By the time the Chief locates Keyes, it's too late.
The weapon turns out to be a sentient parasitic race called The Flood. The Flood decimates the Marines and the Covenant alike, turning their mangled corpses into zombie-like Flood warriors. The Halo is now the stage for a gruesome three-way battle.
Let me Tell You About My Robot Pal
At this point, the Chief is snatched up by Halo's quirky monitoring A.I., Guilty Spark. Spark, who refers to the Chief as "The Reclaimer," tells him that the Flood must be stopped and the only way to do so is to activate Halo's defense system. Spark explains that the Halo is a facility built by a long dead race known as The Forerunner. It was created to study and contain the Flood. Apparently there was a Flood outbreak long ago that was taken care of by the Halo's "contamination protocol". Spark is convinced this protocol is the only way to stop the current outbreak.
With Guilty Spark as his guide, the Chief fights his way through hordes of Flood to retrieve the Index, an ancient key needed to unlock Halo's defenses. After battling his way between Covenant and Flood forces, the Chief returns to the control room to activate Halo.
Once again, Cortana intervenes. Cortana explains to the Chief that he has been misled by Spark. If the Chief activates the Halo, he will destroy all the life in the galaxy. He learns that it isn't that the Halo holds a weapon; the Halo is a weapon. The Halo fires a massive pulse into space that wipes out all organic life that is capable of acting as a host to the Flood -- humans, Covenant, squirrels, everything.
Burning Down the House
After learning the Truth about the Halo, the Chief and Cortana decide the only thing to do is destroy it before it's activated by someone or the Flood figure out a way to get off of it.
Now Chief is fighting The Covenant, the Flood, and the A.I. Guilty Spark and his robot drones. He sneaks onto a damaged Covenant ship that the Flood are trying to escape in and retrieves the Personal Data Transmitter (PDT) from Captain Keyes' badly deformed body. Keyes' PDT is the key the duo needs to authorize the self-destruct sequence on the crashed Pillar of Autumn. The subsequent explosion caused by the PoA's reactors will destroy the Halo and everyone on it.
The Chief and Cortana make their way to the PoA crash site and fight corridor-to-corridor to get to the reactor chamber. They start the self-destruct sequence, and, as the ship explodes around them, they narrowly escape annihilation by hopping aboard a Marine starfighter docked in the PoA's hanger.
As the Halo explodes in space, Cortana scans for life in the surrounding area. No Marines, no Covenant, no Flood -- nothing but "dust and echoes." Just as before, there are no survivors but the Chief. The chief now must make his way back to Earth. As the Chief and Cortana speed away, Guilty Spark flies away into space unnoticed muttering to himself...
Source: Halo novels The Fall of Reach and The Flood