The full story
The film opens with Thor hanging in chains, bantering with the fire demon Surtur about the prophecy of Ragnarok. He defeats Surtur, takes his crown, and heads home to find Asgard in disarray. Loki has been impersonating Odin for years, building statues of himself and commissioning plays about his fake death. Thor is not amused.
The brothers find Odin in Norway (with an assist from Doctor Strange). Their father is dying, and with his last breath he warns them: his firstborn is coming. Hela, the Goddess of Death, was Odin's original weapon of conquest—his executioner. He imprisoned her when her ambition grew too vast. Now she is free.
Hela destroys Mjolnir with one hand. The hammer that defined Thor for two films is shattered. She sends both brothers tumbling through space and conquers Asgard, resurrecting her army of the dead and her giant wolf Fenris.
Thor lands on Sakaar, a garbage planet ruled by the Grandmaster. He is captured by Scrapper 142—a hard-drinking warrior who turns out to be a Valkyrie, last survivor of Asgard's legendary female fighting force. She sells Thor to the Grandmaster's gladiator arena, where he will face the champion.
The champion is the Hulk. Banner has been stuck in Hulk form for two years since Ultron, fighting in the arena. Thor recognizes his "friend from work" and they fight. Thor needs Banner, not Hulk, to escape Sakaar and save Asgard.
Loki is already on Sakaar, scheming as always. When Thor gives him the chance to betray him again, Loki chooses his brother. "I'm here." Their relationship evolves past the cycle of trust and betrayal.
Thor realizes he cannot defeat Hela—she is too powerful, and Asgard makes her stronger. Ragnarok is not something to prevent but rather the solution. He has Loki resurrect Surtur, and the fire demon destroys Asgard entirely, taking Hela with it.
"Asgard is not a place. It is a people." Thor becomes king of the refugees, leading his people toward Earth on a ship full of survivors. He has lost his hammer, his eye, and his home. He understands what it means to be worthy—not through the weapon, but through the choice to sacrifice everything for his people.
In the post-credits scene, Thanos's ship appears.