After what happened in Sokovia during the battle of Ultron; Thor was fighting to liberate and achieve peace again in the nine kingdoms of Asgardian cosmology.
Thor is captured by Surtur, leader of the fire demons of the planet Muspelheim, who tells him that Ragnarok's prophecy is about to be fulfilled, and that his father God Odin was no longer in Asgard.
Using the Mjolnir Thor, he frees himself and confronts Surtur, defeating him and taking the crown of fire to take her to Asgard's vaults, invoking Heimdall, guardian of the Bifrost; Upon arriving in Asgard, he observes that things have changed.
He discovers that Loki, who he considered dead, pretends to be Odin and reigns fully in Asgard. Thor forces him to confess, Odin is on the earth held in a care home. Upon arriving on earth they are intercepted by Dr. Stephen Strange; who transports them to the shores of Norway to visit their father.
Odin tells them that the end has come, their permanence on the earthly plane is about to end, but that a great problem lies with it, the terrible Hela will be released from her prison, warns them that her power lies in Asgard and it will be impossible to defeat her. Odin dies and as predicted, immediately appears the powerful Hela, who introduces herself as Odin's firstborn and queen of Asgard.
Thor attacks with the powerful Mjolnir, Hela takes it with one hand and breaks it like a cookie demonstrating his immense power, dismayed Loki asks Skurge to transport them with the Bifrost, but they don't count on Hela also being part of the trip. During the chase inside the bridge, Hela expels them to space and ends up on the planet Sakaar dump, destiny of the wormholes.
There it is captured by the collector 142, who submits it by placing an electrical sensor of obedience. He sells it as a fighter to the Governor of Sakaar, who promotes the death fights within the colosseum. There he finds Loki, but he doesn't want to mix with Thor because he can ruin his good relationship with the Governor.
Thor is locked in the dungeons where he awaits his turn to fight the great champion of the governor, an invincible being who destroys all his opponents and has never been defeated.
When the time comes, Odin's son shows up in the area waiting for his dreaded opponent; when the doors opened and the champion came out, Hulk turned out to be Hulk, if Hulk, the powerful green giant member of Avengers.
Thor explodes with joy to see his battle partner, but Hulk is not so happy and prefers to fight causing the awakening of the true power of the God of Thunder.
But how did Hulk get to Sakaar? Remember that at the end of "Avengers Age of Ultron" Hulk took the Quinjet and fled aimlessly for not being able to be with Natasha in any way he wanted, which caused him to go to space and enter a wormhole and reach the receiving planet of everything lost.
Thor ragnarok is the film that opens the window to the final stage of phase three of the MCU, immediately giving way to “Avengers Infinity War” where we will see Thanos collect the six gems of the infinite and trigger a series of crucial events for the survival of life itself.
Marvel has been building this whole universe of superheroes and fantasy that excites young people and adults for just over ten incredible years.