What is the story of halo?

Answer 1

Basically, humanity is fighting a losing war against the Covenant.

More detailed:

An alliance of seven alien races, and several sub species, colloquially known as the Covenant, or Holy Alliance, embarks on a crusade against the Human species, seeing us as to erratic and unpredictable, we posed a serious threat to the mission of the Covenant, to find and detonate the Halo rings, ancient artifacts left behind by an alien race known as the Forerunners which the Covenant believe is their gods. The Covenant believe that by detonating the Halo Rings they will embark on the 'Great Journey' and become gods. Of course this is not true, so the Humans try to stop the aliens, although most of their species are mush more powerful than humans, so to cope with this threat, the UNSC, the United Nations Space Command, and ONI, the Office of Naval Intelligence, kidnap children specially qualified, and insert them into a special military program known as the SPARTAN program. Master Chief, or John-117 is plucked from his home on Eridanus IV, a planet in the same solar system as Reach, and a clone is put into his place, the clone is engineered to die of natural causes shortly after, so the family suspects nothing. After the children are put through vigorous physical and mental training, they are biologically augmented, so they become as strong as the warrior class of the Covenant, the Elite. 70% of all recruits don't live past the augmentation unfortunately. The SPARTAN Program was started to deal with rebels, but when the Human colony of Harvest was attacked by the Covenant they were put into action. Master Chief is early on deployed on the Pillar of Autumn to assist in the evacuation of Reach, and during the events of Halo: Reach he is secured in a cryogenic chamber.

After the Pillar of Autumn makes a blind jump into space, according to the Cole Protocol which states not to lead any Covenant vessels to Human population centers, they discover the first Halo, Installation 04. With the help of the Ship's AI they manage to find a map room for the alien ring that the Covenant call: The Silent Cartographer, after an assault on the map room they discover the location of Halo's control room, they manage to destroy the entire ring, but not before the Master Chief encounters a dark evil. Locked away on the ring world was an ancient parasite, named: The Flood, and when the Covenant tried to gain access into the Control Room, they released the Flood, but were able to seal the doors to the outside shut, until a human assault team opened the doors and let the Flood loose.

After the Halo ring was destroyed, Master Chief returned to Earth to be accommodated on his victory, at the same time, a Covenant Shipmaster named: Thel 'Vadamee was being tried in front of the Hierarchs, the Prophets: Truth, (Ord Casto) Regret (Lod Mron), and Mercy (Hod Rumnt) for Heresy against the Covenant, for letting a small human bomber piloted by the Master Chief get away, he was found guilty and was striped of his rank, later he would become the Arbiter, tasked with the objective to kill Master Chief, at this time an award show was taking place on the UNSC Orbital Defense Platform Cairo, and the Prophet Regret was leading an invasion on Earth, though they did not know Earth was our home planet, they just knew Earth held the Portal to the Ark. After a fierce battle, and after the city of Mombasa in Kenya was destroyed by a Covenant Capitol ship jumping into slipspace, but Captain Amanda Keyes, the daughter of the highly decorated Jacob Keyes (Captain of the Pillar of Autumn) managed to follow the ship to Installation 05, or Delta Halo. The Humans managed to launch an attack on the ring, led by the Master Chief he soon discovers, partnered by the Arbiter whom went to Installation 05 to kill the Chief, they discover the Gravemind, the hive controller of the Flood he reveals that the Forerunners created the Halo Array to kill off the Flood's food thus starving out the Flood, and it is then that the Arbiter returns to tell his leader, the Great Journey is a lie, that if Truth is to activate the rings, all life will be exterminated. After this, the Gravemind told the Master Chief he had to stop the Covenant, because they were planning on embarking on the Forerunner dreadnought, a huge monolithic battle ship seen as a religious emblem. So, the Gravemind teleports the Master Chief into High Charity, or the Covenant Holy City, where he tries to destroy the power cable connecting to the Dreadnought, this was a failed attempt, and shortly before he finds the control panel to un-hook the power transfer cable, a flood infected Frigate comes into the city and crash lands into the side of High Charity, Master Chief quickly grabs onto one of the magnetic rings transferring power from the ship to High Charity, and leaves Cortana behind on the control panel, a necessary sacrifice, Master Chief then gets a glimpse of the battle being raged outside of High Charity, as the Elite forces launch quick and desperate attacks, it's not enough to subdue the Prophet's progress. Master Chief stown away on board rides to Earth.

In the Beginning of Halo 3, you can see a piece of debris flying off of a meteor looking object, that is the Chief who just ejected himself quite manually from the Forerunner ship, and after he crash lands, his squad gets orders to destroy a Covenant anti-air cannon in the Kenyan city of Voi, Master Chief can clearly see the havoc the Covenant are bestowing upon the Kenyan Wild Life Preserve, a large hole had been dug out, revealing the amazingly large Forerunner Portal structure underneath, with the Forerunner dreadnought sitting in the middle, acting as a control stand, and key to open the portal, and due to the huge amounts of technology being unearthed, a large electromagnetic storm begins to form above the portal, and when the portal was activated, it was like a beacon, almost instantaneously all Covenant fled Earth, and arrived at the Ark. It took the Human-Sangheilli alliance a while to contemplate weather or not to proceed through the Portal, and they finally decided this was there last hope, so they proceeded and discovered the massive 8-armed Installation 00 (The Ark, which in ancient times, served as an Ark for the species to hind in case of the rings detonation). The Elites immediately attacked the orbital Covenant vessels, leaving it to the more than capable humans to find the Cartographer, and thus find the Control Room, it was shortly after that both were found, and it was revealed that the Ark not only served as a temporary home, but also as a control center to remotely activate ALL of the Halo Rings, the Humans and Elites quickly embark on a head-on assault dubbed "Operation Trident" to kill Truth and stop the war. After they take over the Control Room, they form a temporary alliance with the Flood, both side knowing they can't win this alone, they both proceed to kill Truth, and after his death, the newly entombed Gravemind steers High Charity into a collision course with the Ark. After Battling the Flood Master Chief, and the Arbiter go down to a small control room where the 343 Guilty Spark eagerly greats them, the Master Chief then puts his hand out to activate a button, which deploys Installation 04B, an incomplete replacement for the first Installation the Master Chief destroyed in Halo CE, the Arbiter and Master Chief quickly decide, that the only way they could get rid of the local flood infestation was to activate the ring (Which is out of range of the other installations, assuring the protection of the Milky Way), so they decide to attack, but first: Cortana. The Gravemind had successfully crash landed High Charity onto a plains biome on the Ark, which is where Cortana was last seen, so they travel to the Ark, take back Cortana, activate the Ring, and as the explosion ripped the Frigate, which was acting as their escape route, apart only half of the ship managed to use it's slipstream drive to escape to Earth, the part the Arbiter was on. Halo 3 ends with a scene of Master Chief floating through the crippled ship's cryogenic freezing room, and before he enters Cryo, he tells Cortana to call him when she needs him.

Halo 4 begins with Cortana screaming "Chief! Chief! John I need you! John!" as he awakens to find the ship being pulled in by a Forerunner planet known as a shield world.