Destiny is known for having a somewhat confusing plot, so if you're a returning player coming back to Destiny 2, you might be fuzzy on the details--and if you're new, it might be really intimidating to jump into. Either way, don't worry; from vanilla Destiny through each expansion, we've covered everything you need to know about the story missions so far.
Here we've outlined the main beats of your Guardian's story, leaving out a lot of superfluous details and information from the Grimoire, so you can get the basics down before starting Destiny 2.
Long before the events of the game was the Golden Age, when mankind discovered the Traveler. The gigantic, spherical entity was terraforming planets throughout the solar system, though its true nature--sentient being, vessel, deity, or something else entirely--is unknown. Its discovery launched humanity into an era of prosperity, as it allowed them to colonize other planets. Top scientists founded the Ishtar Academy on Venus, and elsewhere people developed Exos, machines with human consciousness. The many technological advancements during this time included that of powerful AI called Warminds, one of which remained through Destiny's present day: Rasputin.
One day, a dangerous enemy of the Traveler known as the Darkness approached Earth, destroying nearly everything in its path and wreaking havoc upon what humanity had created. This is referred to throughout Destiny as the Collapse. The Traveler sacrificed itself to save humanity, using its Light to drive the Darkness away and create the personal AI called Ghosts that assist Guardians. After the Collapse, the Traveler remained hovering dormant over Earth, and the survivors built the Last City within its limited protection. This final populated city is where the Tower--home of the Guardians--is located.
Another threat faced the City: an alien race known to humans as the Fallen, who wanted the Traveler for their own.
Much of vanilla--also known as Year One--Destiny's story consists of setup for things to come.
At the beginning, you're a Guardian who has been dead for some time and is then revived by a Ghost. You wake in the Cosmodrome, an ancient spaceport in Old Russia, Earth. Like other Guardians, you can wield the power of the Traveler's Light. Your mission is to fight past the Fallen here and activate an old jump ship to escape to the Last City.
Once at the Tower, you learn about the Vanguard, who protect the City. It consists of Commander Zavala, Cayde-6, and Ikora Rey, who represent each of the three playable classes (Titan, Hunter, and Warlock, respectively). You also learn of the Traveler through someone called The Speaker, a representative of the Traveler who helps to guide Guardians against the Darkness. Your job as a Guardian is to help protect the Last City from any threats.
You return to Old Russia to recover a drive for your ship that will allow you to travel to other planets in the Solar System. While there, you encounter the Hive, an old species that worships the Darkness. You fight past the Fallen and the Hive and open an Array that gives you access to more technology and information. Through this you discover the Warmind Rasputin, thought to be long-dead.
You protect Rasputin from the Fallen and head to the Moon in search of the last Guardian that had gone there. In the process, you wake up the Hive (again). You meet an Exo known as the Stranger who tells you to meet her on Venus if you survive.
On Venus, you learn about a complex, highly intelligent machine race called the Vex. The Exo Stranger shows up after you push back the Vex in the Ishtar Academy (but her story is largely nonsensical, so don't worry about it too much). From here you travel to the Reef, a sovereign collection of space trash and asteroids, and meet the Awoken Queen of the Reef, Mara Sov. You learn that the Fallen that make their home in the Reef--the only Fallen in the game who aren't your enemies, since they serve the Queen--belong to a sect called House of Wolves.
Back on Venus, the Fallen House of Winter has enlisted the help of a Fallen Archon Priest they sprung from the Queen's Prison of Elders. In a Strike mission, you defeat him and protect the knowledge kept inside the Ishtar Academy.
Later, you go to Mars, home of the warmongering rhino-like race known as the Cabal. Your mission is to destroy the Heart of the Black Garden, an area on Mars built by the Vex. In doing so, you've taken another step to save the Traveler.
At the end of vanilla Destiny, The Stranger gifts you with her weapon, The Stranger's Rifle.
The Raid, Vault of Glass, involves an area where Vex have control over time and space. In the Vault of Glass, all timelines converge--there you find corpses of people who are long dead, but because reality is bent here, parts of them and their memories still exist. The Raid involves the battle and then defeat of Atheon, who is said to be the point where all of space-time converges.
Expansion 1: The Dark Below
After you've destroyed the Black Garden's heart, a former Guardian named Eris Morn arrives in the Tower.
Lurking below the surface of the Moon is Crota, Son of Oryx and Hive prince, who wields the power to destroy the Guardians' Light. (We'll come back to Oryx in a bit.) Eris Morn is the only survivor of a failed assault on Crota, so she asks you for help--his followers are working to bring about his return. You defeat his disciple Omnigul, a Hive Wizard, before destroying Crota's essence and stopping the attempted resurrection.
In the Crota's End Raid, you summon, fight, and defeat Crota himself.
Expansion 2: House of Wolves
Remember the Reef and its Queen? The Fallen House of Wolves had served Queen Mara Sov for some time, including when you first met her. But Skolas, a Fallen captain, is attempting to unite the fragmented Fallen sects under one House, and under his command, the House of Wolves betray Mara Sov. One Fallen, Variks of House Judgment, remains loyal to the Queen.
Your job is to hunt down Skolas and prevent the formation of a united Fallen front. You head to Venus, where he is studying Vex technology, to find him. You capture him alive and send him to the Prison of Elders, then later head there and defeat him again for some reason.
Expansion 3: The Taken King
The Taken King expansion is widely regarded as the high point of Destiny's storytelling. The writing feels more cohesive, it strikes the right tone, and the characters have more presence than in previous iterations.
Oryx is the founder and God-King of the Hive, having been reborn from a pact with the Darkness. He arrives in the Solar System looking to take revenge on the Guardians for the death of his son, Crota. He rolls into the rings of Saturn on his ship, the Dreadnaught, which comes equipped with weapons of mass destruction. Oryx's army consists of the Taken, other enemy races that he controls and distorts by sending them into and back out of another dimension. They resemble shadowy versions of existing enemies, but they move as if they're glitching in and out of reality.
The Reef attack Oryx's army on Saturn in a suicide mission designed to help you make it onto the Dreadnaught. In the process, the Queen and all her forces are wiped out by the ship's super weapon. The Cabal are already there when you arrive, also engaged in a battle, but they are in the process of retreating from the Taken. You then have to recover special tech from Cayde-6 in order to sneak onto the ship, where you disable its weapon and, after several more missions, take down Oryx.
But Oryx only retreats--you defeat him fully in the Raid, King's Fall.
Expansion 4: Rise of Iron
The Fallen House of Devils started looking in Old Russia for something they could use to improve themselves body and mind. There they found SIVA, an ancient nanotechnology with the ability to make them more powerful. Those who use it are called Splicers.
Lord Saladin of the nearly extinct Iron Lords--a group formed after the Collapse with the purpose of uniting humanity--calls on you to investigate and secure the Iron Temple, which is under attack by a rebuilt Sepiks Prime (an enemy you fought for the first time in a Year One Strike). There you learn of the Fallen Splicers and enter the vault where they are replicating SIVA in order to stop the spread.
In the Wrath of the Machine Raid, you defeat the Splicers and their leaders.
That brings us to Destiny 2. The sequel begins two years after the events of the original; the Tower has been destroyed and The Last City is under attack by a Cabal faction called the Red Legion, led by Ghaul. Humanity has been pushed out of their last safe haven, and your job is to reclaim what you've lost.
Key characters from what we've seen so far include the Vanguard (Zavala, Cayde-6, and Ikora) and shipwright Amanda Holliday, who were little more than quest-givers and gear merchants in the first game. Check out the full Homecoming story mission to see how they've taken center stage.