Two years after the Battle of Sokovia, Thor is imprisoned by the fire demon Surtur, who reveals that Thor's father Odin is no longer on Asgard. He explains that the realm will soon be destroyed during the prophesied apocalypse Ragnarök, once Surtur unites his crown with the Eternal Flame that burns in Odin's vault. Thor frees himself, defeats Surtur, and takes his crown, believing he has prevented Ragnarök. Thor returns to Asgard to find Heimdall gone and his estranged brother Loki posing as Odin. He forces Loki to help find their father, who Loki put a spell on after his presumed sacrifice years before.

With directions from Stephen Strange, they find Odin in Norway. Unpossessed, Odin explains that he is dying, Ragnarök is imminent despite Thor's efforts, and his death will free his firstborn child, Hela, from Hel, a prison she was sealed in long ago. Hela, a sibling Thor and Loki did not know existed, was the leader of Asgard's armies and conquered the Nine Realms with Odin, but he imprisoned her and wrote her out of history after she became too bloodthirsty and did not share his views on peace.

Odin dies and Hela appears, destroying Thor's hammer Mjolnir. She pursues the two as they attempt to flee through the Bifröst Bridge, knocking them out into space. Arriving in Asgard, Hela defeats its army, slays the Warriors Three, and resurrects the ancient dead who once fought with her, including her giant wolf Fenris. She appoints the Asgardian Skurge as her executioner. Hela plans to use the Bifröst to expand Asgard's empire, but Heimdall takes the sword that controls the Bifröst.

Meanwhile, Thor crash-lands on Sakaar, a garbage planet surrounded by wormholes. A slave trader designated Scrapper 142 subdues him with an obedience disk and sells him as a gladiator to Sakaar's ruler, the Grandmaster, with whom Loki has already ingratiated himself. Thor recognizes 142 as a Valkyrie, one of a legendary force of female warriors who were killed fighting Hela eons ago. Thor is forced to compete in the Grandmaster's Contest of Champions, facing his friend Hulk. Summoning lightning, Thor gets the upper hand, but the Grandmaster sabotages the fight to ensure Hulk's victory. Still enslaved after the fight, Thor attempts to convince Hulk and 142 to help him save Asgard, but neither is willing.

Thor finds the Quinjet that brought Hulk to Sakaar. A recording of Natasha Romanoff causes Hulk to transform back into Bruce Banner for the first time since leaving Sokovia. 142 decides to help Thor save Asgard. Loki helps them steal one of the Grandmaster's ships. They liberate the other gladiators who, incited by two aliens named Korg and Miek, stage a revolution. Loki again attempts to betray his brother, but Thor anticipates this and incapacitates him. Thor, Banner, and 142 escape through a wormhole to Asgard, where Hela's forces attack Heimdall and the Asgardian citizens. Hulk defeats Fenris; Loki and the gladiators arrive to help, and a repentant Skurge sacrifices himself to save the citizens.

Thor, battling Hela, loses his right eye and has a vision of Odin that helps him realize only Ragnarök can stop her, as she is too powerful. He sends Loki to trigger Ragnarök by placing Surtur's crown in the Eternal Flame. Surtur is reb…

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While promoting the release of Thor: The Dark World in October 2013, Chris Hemsworth expressed willingness to portray Thor for as long as "people wanted more", adding that he was contracted for another Thor film and two more Avengers films. Producer Kevin Feige stated that the next Thor would build from elements at the end of The Dark World. In January 2014, Marvel announced that Craig Kyle and Christopher Yost would write the screenplay for a third film, with Feige again producing; the story was being outlined that July. At the end of October 2014, Feige announced that the film would be titled Thor: Ragnarok, with a scheduled release date of July 28, 2017. Hemsworth and Tom Hiddleston were set to return as Thor and Loki, respectively. Hemsworth earned $15 million for the film. Feige added that the film would be "very important" in Phase Three of the Marvel Cinematic Universe (MCU), and also confirmed that, in the context of the film, the word Ragnarok means "the end of all things". He felt that "people will [not] read into that title alone what the movie's going to be."

In February 2015, Marvel pushed back the release date to November 3, 2017. That April, Feige was expecting a draft for the film soon, and a month later he stated that a director, additional screenwriter, and further casting announcements would be revealed "towards the end of the summer", with filming set for June 2016. The Dark World director Alan Taylor explained that he would not be returning, as "the Marvel experience was particularly wrenching because I was sort of given absolute freedom while we were shooting, and then in [post-production] it turned into a different movie. So, that is something I hope never to repeat and don't wish upon anybody else." During the 2015 San Diego Comic-Con, Jaimie Alexander said that she would reprise her role as Sif in "a very pivotal part" of the film.

By October 2015, Taika Waititi had entered negotiations to direct Ragnarok. Other directors under consideration included Ruben Fleischer, Rob Letterman, and Rawson Marshall Thurber. Elizabeth Banks had expressed interest in presenting a pitch for the film, but never ultimately met with Marvel to do so. Marvel presented the prospective directors with "the ten different ideas that we had for the movie", asking them all to come back with a clearer picture of what the film should be. Waititi created "a sizzle reel for the tone, and some joke stuff" using clips from other films, including Big Trouble in Little China (1986). Despite being a discouraged practice within the company, Marvel considered Waititi's reel to be "amazing", particularly its use of Led Zeppelin's "Immigrant Song" for the score, which Feige felt "defined what Taika was going to do with this". The song was later used in the film itself, and for marketing it. On why he decided to pursue directing the film, after stating in 2012 that …

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By August 2016, Mark Mothersbaugh was hired to score the film. The synthesized score is influenced by the work of Jean-Michel Jarre. Waititi stated he would have asked the band Queen to work on the soundtrack for the film if their lead singer Freddie Mercury was still alive, because the film is "a cool, bold, colorful cosmic adventure" which would have suited the "feel" of the band. Additional music featured in the film include "Immigrant Song" by Led Zeppelin and "Pure Imagination" from the film Willy Wonka & the Chocolate Factory (1971). Patrick Doyle's themes from Thor and Brian Tyler's themes from The Dark World and Avengers: Age of Ultron, as well as Joe Harnell's "The Lonely Man" theme from The Incredible Hulk series, are also used in the film. Hollywood Records released the film's soundtrack digitally on October 20, 2017, and physically on November 10, 2017.