Hiddleston recently briefly recapped Loki's previous adventures, but let's take a more detailed look at his past so we can figure out where he is before his show kicks off.
Who is this Loki guy?
First seen in 2011's Thor, Loki is the son of Frost Giant king Laufey, who abandoned Loki as a baby. Fortunately for Loki, Asgardian king Odin found the wee lad and adopted him -- he was then raised as an Asgardian prince along with Odin's biological son Thor.
Loki wasn't quite the warrior Thor was, and grew up in his first-born brother's shadow. But he grew close with his adoptive mother, Frigga, who taught him magic so he'd have a slightly more tricksy skill set. What could go wrong?
He's pretty jealous of Thor, so he plots to usurp the Asgardian throne and learns of his true heritage along the way. He ends up killing Laufey, then battles Thor before falling into a wormhole. Plan foiled, but he did learn about his past.
Loki cunning scheme success level: 3/10
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At this point, Loki enters the service of Thanos as the alien villain is gathering the Infinity Stones for his dastardly plot to wipe out half of all life in the universe. He's tasked with staging an alien invasion of Earth retrieving the Tesseract (Space Stone) for his new boss in 2012's Avengers.
This threat brings the Avengers together and ends with Loki being defeated again at the Battle of New York. He's brought back to Asgard to face justice for his many crimes.
Loki world domination success level: 2/10
Seizing the Asgardian throne
In 2013's Thor: The Dark World, Loki joins Thor to stop the Dark Elves after they kill Frigga and appears to heroically sacrifice himself to save his brother's life. It's all smoke and mirrors though -- Loki actually shapeshifts into Odin's form and takes control of Asgard, with the real Odin placed under a spell and banished to Earth.
It all falls apart in 2017's Thor: Ragnarok, and the real Odin reveals the existence of Thor and Loki's sister -- the very mean Hela -- as he dies. Released from a long imprisonment by her father's death, she humiliates her brothers before booting them out of Asgard and taking the throne for herself. To stop Hela, Thor kicks off a series of events that leads to Asgard's destruction. Loki joins the survivors and flees to Earth on a spaceship.
Loki redemption success level: 8/10
Thor, now king of the Asgardian refugees, reconciles with Loki, but their newfound brotherly love is short-lived. In the opening moments of 2018's Avengers: Infinity War, Thanos and his goons intercept the Asgardian ship while hunting for the Tesseract. Loki uses his trickery to try to stab Thanos, so the Titan murders Loki.
"No resurrections this time," says Thanos, before dropping Loki's lifeless corpse to the deck.
This heroic sacrifice appeared to be the heartbreaking end of the God of Mischief's story, but the Avengers soon change that with some timeline meddling.
Loki Thanos-killing success level: 0/10
Rise of the Loki Variant
So 2019's Avengers: Endgame saw the heroes travel back through time to retrieve the Infinity Stones to undo Thanos' genocidal Snap. One group goes to the Battle of New York, but totally screws up their attempt to get the Tesseract. In the chaos, an opportunistic Loki steals the Tesseract and teleports away -- a move that presumably created an alternate timeline branching off from the end of the first Avengers movie.
This leaves us with a Loki who hasn't gone through the character development of his slain counterpart. The Disney Plus show follows this version of the character, who'll presumably be meaner and more arrogant than the redeemed version who died in Infinity War, after he falls into the clutches of an organization of chronological cops known as the Time Variance Authority.
Loki's Endgame escape "broke reality," according to TVA agent Mobius, and the timeline bureaucrats force him to make it right -- hopefully he'll have to grapple with the changes his OG MCU counterpart went through and figure out what kind of Loki he's gonna be. Get excited, I reckon we're in for a wild ride.