The first game of Microsoft's immensely popular trilogy is called Halo: Combat Evolved. The premise of the game is this: the human race has been in a decades-long battle with the Covenant, which is a collective group of various alien species. The Covenant is waging holy war and desires to wipe out the human race.
Following a devastating attack by the Covenant on the stronghold planet of Reach, only one ship, the Pillar of Autumn, escapes by making a blind jump. However, this jump lands them in close proximity to a Covenant artifact. This artifact is an artificially built ring called Halo, which is habitable and planetary in size. Built by an ancient race called the Forerunners, and unbeknownst to both the humans and the Covenant, Halo was a weapon of incredible power.
As the Autumn came under attack, one of the last surviving SPARTAN-II super soldiers was activated and charged with safeguarding Cortana, the shipboard AI. This was Spartan 117, known more commonly as the Master Chief.
With Cortana's assistance, the Master Chief and the surviving marines rallied to fight the overwhelming Covenant resistance on the ring's surface, and raced to unlock the secrets of Halo.
As the story unfolds, it is revealed that the Covenant believed that by activating the ring a "divine wind" would be released and would cleanse the universe of heresy. This ring was one of seven installations. Contrary to the Covenant's beliefs, the rings were a final solution against a threat so awful that the Forerunners who built the ring thought killing an entire galaxy was the only solution.
This threat is the Flood, which is a parasitic life form that is both intelligent and powerful. Humans and Covenant alike were infected and assimilated into their ranks. In order to stop this threat, the Chief, much to the chagrin of the Covenant, blows up the reactor core of the Pillar of Autumn, thereby destroying the ring instead of activating it. Master Chief knows this is just the beginning of the fight.
Halo 2 capitalizes on the success of its predecessor. The story this time is split between the Master Chief and an Elite (one of the Covenant's many races) commander. This Elite commander was seemingly sentenced to death for failing to prevent Halo's destruction. However, he is secretly ordered by the Covenant's High Prophets to take Covenant's sacred mantle of the Arbiter, which is essentially a symbol for "blade of the Prophet's will." The role of the Arbiter is expected to be a death sentence as well.
The story begins with Master Chief receiving awards for his work on Halo. However, the ceremony is cut short due to a Covenant invasion.
Master Chief is charged with stopping the Covenant strike on Earth. Meanwhile, the Arbiter was tasked with assassinating a "heretic" (as labeled by the Prophets) Elite spreading "lies" about the true purpose of the sacred rings. After completing his mission, the Arbiter starts to question his loyalties to the Covenant.
The stories of Master Chief and the Arbiter converge with the release of the Flood from Delta Halo, a second installation built by the Forerunners.
Things get convoluted when mysterious Flood creature called the Gravemind manipulates both the Arbiter and Master Chief because they all have a "common" goal: To prevent the rings from being activated. Gravemind sends the Arbiter and Master Chief in separate directions to prevent The High Prophets from activating Delta Halo.
Master Chief goes to the Covenant city called High Charity, an immense space station, and pursues the remaining Prophets. During his mission, he finds himself in the middle of a Covenant civil war between the Brutes and the Elites (who were the former protectors of the Prophets).
After capturing a Covenant ship, the Flood arrives at the city and begins infecting the population. The only remaining High Prophet, Truth, escapes on a ship located in the core of High Charity. The Master Chief stows away on board and leaves Cortana behind in order to destroy Delta Halo and High Charity if the Brute captain Tartarus activates the ring.
The Arbiter, with the help of fellow Elites and human marines, manages to stop the ring from being activated. However, the unexpected shutdown causes the ring to send a signal out to the other remaining Halos, sending them all into a "standby" mode so they can be remotely activated from the Ark, which is a device that can activate all of the remaining rings.
Meanwhile, the ship that Master Chief is stowed away on approaches Earth. The game ends with the Chief being questioned by one of Earth's patrol services regarding what he is doing aboard the ship. The Chief answers by stating, "Sir, finishing this fight, sir."
