It may come as a surprise to some that Halo actually has a single-player component. Thousands of Halo fans play the games solely for the competitive multiplayer and will buy the game with no intention of ever playing through the game's main story.

Frankly, that's a shame: Halo boasts a surprisingly strong story, and the teams at Bungie and 343 Industries have crafted one of the most detailed science fiction universes out there. Sure, there are a few elements of the story that ape other sci-fi properties, but for a game that started out as nothing more than "man in green armor kills everything," Halo has a surprisingly rich lore. Seeing as how Halo 5: Guardians is set to continue the saga later on this year, we thought it'd be a good time to look back at the adventures of the Master Chief.

For the sake of brevity, we'll only be looking at the events of the main games in the series: trying to catalog everything about the Halo Expanded Universe would require its own encyclopedia. If you've been wondering about what's going on in the games, or simply want to brush up on what you've already played through, here's everything you need to know about the story of Halo.

The Characters

Master Chief

Master Chief Petty Officer John-117 — or "Master Chief" for short — is the hero of the Halo franchise. As a genetically-modified SPARTAN II super-soldier, the Master Chief is basically a walking tank. Unfortunately, he's also one of the last of his kind, as most of the other SPARTANs were wiped out before the first game in the franchise.

Cortana

The Master Chief is powerful on his own, but he's even more lethal once Cortana is at his side. The artificial intelligence is basically the Chief's co-pilot, and Cortana is one of the most advanced pieces of technology that humanity has ever created. In fact, she's one of the main reasons that the Master Chief is still alive today.

The UNSC

The United Nations Space Command is the governing body of the Halo universe. Following the wars of present-day Earth, the UN reforms into the UNSC once extraterrestrial human colonies are established. The military makes up only a small part of the organization, though it's the most explored branch within the Halo lore.

The Covenant

A conglomerate of alien species bent on humanity's destruction, the armies of the Covenant are almost unstoppable. Powered by ancient alien technology and fanatical religious beliefs, the Covenant believe it is their duty to wipe out humanity — and without the help of the Master Chief, they probably would have succeeded.

Halo

Halo isn't just a location, or a giant space station: it's a massive extraterrestrial weapon, built for use on a galactic scale. It is the ultimate last resort, though the station also acts as the prison for one of the deadliest secrets to ever threaten the galaxy.

The Flood

There is only one threat to humanity greater than that of the Covenant, and it is called the Flood. The parasitic species kills and rewrites its host's DNA upon infection, then uses its new form to spread the infection further. While the Flood's combat forms are similar to zombies, they're also faster, stronger and deadlier than any shambler ever could be.

The Forerunners

For a long, long time, little was known about the Forerunners. They were an ancient, galaxy-spanning empire with access to technology beyond humanity's wildest imaginiation — until they disappeared from the galaxy without a trace. All that remains are ancient relics and long-forgotten weapons from another age ...

The Story Thus Far

The story of Halo begins roughly 40 years before the events of the first game. John, a young boy living on one of Earth's many interstellar colonies, is abducted from his home and inducted into the UNSC. He is one of the first subjects in the SPARTAN-II Project: a covert operation to create the greatest soldiers to ever walk the battlefield. For years, John and his fellow subjects are molded into highly-trained and lethal soldiers who know nothing but the art of war. John-117 is one of the project's most promising subjects.

At 14 years of age, John and the other SPARTANS undergo dangerous and horrifically painful genetic augmentations. The children would be gifted with enhanced strength, speed and reflexes — unfortunately, the procedures physically disabled or killed many of the other subjects. Of the original 75 recruits, only 33 made it through the augmentation process unscathed.

In the following months, humanity would make first contact with the Covenant army. Entire colonies began disappearing one after the other, and any direct engagements would always end with humanity's armies being annihilated. In an effort to turn the tides, the surviving SPARTANs were given the Mjolnir Mk. IV power armor — otherwise known as the Chief's iconic green armor.

Years of brutal conflict followed: while the SPARTANs were able to hold the Covenant ground forces back, capital ships would raze entire planets from orbit. Humanity was fighting a losing battle, and the UNSC knew this. A desperate, last-ditch plan was hatched: capture one of the Covenant leaders (known to humanity as "Prophets") and hold it for ransom in an attempt to end the war. Just as the plan was set to be enacted, the Covenant found humanity's greatest military colony: planet Reach.

Halo: Combat Evolved

After barely escaping the planet's destruction aboard the UNSC Pillar of Autumn, the Master Chief was reintroduced to Cortana, his AI companion. The ship and its crew escaped the destruction of Reach, only to stumble upon something far worse: upon exiting Slipspace, the Master Chief and Cortana discovered Halo. However, they weren't alone: the Covenant fleet that had been exploring the gigantic ring-world was quick to shoot the Autumn down.

After crash-landing on the ring's surface, the Master Chief and Cortana quickly discovered why the Covenant was so interested in keeping Halo safe. The ring wasn't just a space station, it was a weapon more powerful than any army could ever dream of. The Master Chief and Cortana began searching for a way to disable the station, but the situation would only get worse from there.

The Flood, a parasitic species that killed, absorbed and rewrote its host's DNA, had been locked deep within Halo — until the Covenant tricked the humans into unleashing it. The Chief, along with the station's AI monitor 343 Guilty Spark, did their best to contain the infection ... but it was too late. Halo was soon overrun with the Flood, and the parasite absorbed most of the Covenant army and the few human survivors that remained.

After fighting his way to the ring's control room, the Chief was ready to activate Halo and wipe out the Flood. It was here that Cortana revealed that the station wasn't built to fire at specific targets — instead, Halo would kill off all sentient life in the galaxy, depriving the Flood of its food. After 343 Guilty Spark turned on them, the Chief and Cortana fled to the Pillar of Autumn's crash site. There, they sabotaged the ship's reactor, and the subsequent explosion tore Halo apart. The two escaped in one of the Autumn's fighters and began the long trek home.

Halo 2

After being reunited with the few survivors that survived Halo's destruction, the Master Chief returned to Earth. Sadly, his homecoming was cut short: after years of keeping the planet's location a secret, the Covenant had finally discovered Earth.

Both the Chief and Cortana quickly joined the UNSC's forces on the ground in South Africa, but by the time they realized the Covenant's plans, it was already too late. One of the orbiting capital ships entered Slipspace just above New Mombasa, and the resulting shockwave completely eradicated the city. The Chief managed to follow the ship through the portal at the last second, only to find something far worse on the other side: Installation 05, otherwise known as Delta Halo.

The Chief, along with a small army of Marines, managed to push the Covenant back and assassinate the Prophet of Regret. After barely escaping the Covenant's reactionary bombardment, he and Cortana were captured by the Gravemind: a Flood intelligence that sought to prevent the Covenant from activating Delta Halo. Along with the disgraced Elite general known as the Arbiter, the Chief and Cortana managed to stop the Halo ring from firing. However, in his attempts to pursue the remaining Prophet, the Master Chief would have to leave Cortana behind ... within the Gravemind's clutches.

Halo 3

After stowing away on the Prophet's ship, the Master Chief returned to Earth. There, he would once again join the UNSC's ground forces. The Covenant had unearthed a massive Forerunner artifact, and the UNSC would launch their counterattack while the Prophets had their backs turned. The Chief made it to the dig site just in time to witness the artifact's activation: a massive portal opened up over the African Savannah, and the Covenant fleet managed to slip through before the UNSC could react. Even with the portal activated, the Covenant quickly became a secondary threat — the Flood had made its way to Earth.

Thankfully, the Chief was able to repel the infection thanks to the Arbiter and his Elites, who had split from the Covenant following the Prophet's betrayal on Delta Halo. With the immediate Flood threat under control, the Chief discovered a message from Cortana, who claimed that there was a way to destroy the Flood without firing the rings. A Forerunner installation known as the Ark laid on the other side of the Covenant portal — along with the Arbiter, the Elites and a small group of Marines, the Chief left Earth yet again.

Once on the Ark, the Elites quickly eradicated what was left of the Covenant navy. On the ground, the Chief fought his way through the remaining Covenant forces but was cut off by an army of Flood just outside the Ark's control room. The parasite claimed that the only way to stop the Prophet was by working together, and it was right — moments later, the Halos were deactivated and the Prophet laid dead at the Arbiter's hand. Of course, the Flood was quick to betray its new allies: without the Halo rings, the Chief and the Arbiter would have to find another way to kill the infection once and for all.

It was then discovered that the Ark had a secondary purpose: it was recreating the Halo that the Chief had destroyed years prior. Activating the incomplete Halo would wipe out the local infestation without firing the other rings, but they would need the station's activation codes — which were being held by Cortana, who was buried within the Gravemind's infected lair. With a little help from the Arbiter, the Chief was able to penetrate the Flood's fortress and rescue his AI companion. They were ready to fire the incomplete Halo — but, as always, it wasn't that simple.

After fighting their way to the new Halo's control room, 343 Guilty Spark (who had been helping the humans navigate the Ark) suddenly turned on the Chief. The AI tried to claim the station as its own, but it was all for naught: Guilty Spark was vaporized and the new Halo was activated. The Chief, Cortana and the Arbiter fled from the crumbling station and were able to escape the Ark's (and the Flood's) destruction by using the same portal they'd come in through.

Unfortunately, only half of the ship would make it back through the portal, as the Ark's explosion caused it to collapse prematurely. While the Arbiter made it back to Earth safely, the Master Chief and Cortana were left drifting through space in the stern of the ship ... and would remain there for the next four years.

Halo 4

It was only after the crumbling remains of the ship drifted towards a massive Forerunner planet that Cortana woke the Chief from his cryogenic sleep, and the pair soon discovered that they had stumbled upon yet another Forerunner installation — and another Covenant fleet. The two were successful in repelling the radical Covenant army that tried to board the ship, but it was too late: the Forerunner installation opened, pulling the ship inside.

Luckily, the Chief and Cortana survived the crash, though the two soon found a far more dangerous foe than the Covenant. The pair accidentally released the Didact, a Forerunner scientist that had once tried to enslave and eradicate humanity's ancestors. The Didact quickly established control over the Covenant forces, though the Chief and Cortana would receive backup of their own: the massive UNSC research vessel "Infinity" arrived after receiving the Chief's distress signal. Once the Didact's forces had been pushed back, the Chief was reunited with the UNSC.

It was here that the Chief learned of a condition known as Rampancy: because Cortana had been active for so long, she was in danger of "thinking herself to death." This combined with the Infinity's captain being more interested in escaping than stopping the Didact, meant that it wasn't long before the Chief and Cortana were left on their own again. Despite this, the duo was able to follow the Didact to Gamma Halo, but they were too late yet again: the Didact was able to retrieve his ultimate weapon and left for Earth.

The Chief and Cortana followed the Didact back to Earth and were able to infiltrate his ship. A grueling battle followed — one that seemingly took Cortana's life. The Chief managed to finally confront the Didact, though the Forerunner's strength was far more than he could withstand. For a moment, it seemed as if the Chief would lose — until a fully-rampant Cortana restrained the Didact just long enough for the Master Chief to attach a grenade to the villain's armor. The grenade incapacitated the Forerunner long enough for the Chief to destroy the ship and its weapons, saving humanity yet again.

However, victory had come at an extremely high cost: Cortana had used the last of her energy protecting the Chief from the ship's destruction. With a final goodbye, Cortana succumbed to her Rampancy and slowly faded away. It wasn't long before the Chief was rescued, but something had changed: Cortana had been his only to link to any real sense of humanity, and she was gone. All that was left was an empty shell.

As far as the games are concerned, no one knows what happened to the Master Chief after Cortana "died." The Halo 2: Anniversary campaign showed that the humans are working with the Arbiter and his troops, and promotional materials have shown that the Chief and Spartan Locke are at odds with one another, but ... what exactly is the Master Chief up to?

It'll still be a while before fans find out: Halo 5: Guardians is set to launch on Oct. 27.

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