When the Hulk returns in Marvel's Thor: Ragnarok, it will be his first appearance in the Marvel Cinematic Universe since the events of 2015's Avengers: Age of Ultron.
As we've seen from the film's marketing, the Jade Giant is pitted against his "friend from work" Thor in a gladiatorial combat by the Grandmaster. But how did the Hulk end up on the planet Sakaar in the first place?
Returning to the finale of Age of Ultron, the Avengers come together to stop the title villain's drone army from attacking the rest of the world. In particular, the Hulk takes out an entire squadron of them by leaping aboard a hijacked Quinjet. The day is saved, until the plane veers toward the stars, and instead of returning home at the request of Black Widow, the Hulk cuts the line and disappears into the stratosphere.
From there, it can be assumed he flew out of the atmosphere in an attempt to find somewhere to live peacefully without fear of the collateral damage he can cause. At some point, he crash-landed on the desert planet known as Sakaar, the setting of Marvel Comics' "Planet Hulk" storyline.
As Screen Rant learned during a visit to the set of Thor: Ragnarok, Sakaar is referred to as "the landfill of the universe," with portals opening to all kinds of debris to its surface. We know Thor finds his way to Sakaar by that means, so it's not difficult to imagine that the Hulk and his Quinjet took a similar route.
At some point, Thor even finds the remains of the craft on the planet's surface. It's probable the Hulk attempted to live among the populace of Sakaar before the Grandmaster turned him into a gladiatorial rock star.
At some point before Ragnarok, the Hulk reverted to a more primitive personality. We know from trailers that he eventually learns to communicate more eloquently than he ever has before, and at some point he will change into Bruce Banner.
Arriving Nov. 3, director Taika Waititi's Thor: Ragnarok stars Chris Hemsworth as Thor, Tom Hiddleston as Loki, Cate Blanchett as Hela, Idris Elba as Heimdall, Jeff Goldblum as Grandmaster, Tessa Thompson as Valkyrie, Karl Urban as Skurge, Mark Ruffalo as Bruce Banner/the Hulk and Anthony Hopkins as Odin.