The movie begins with three scientists, Jane Foster, Erik Selvig and Darcy Lewis, setting up some sort of scanner. Selvig tries to convince Jane that what they are doing is pointless, but Jane says that 17 previous storms had been pinpointed exactly to the second, and that they must mean something. As they speak, a huge whirling cloud appears before them and Jane starts recording it with an infrared camera while they drive. Darcy loses her nerve and turns the car away when they get near; Jane grabs the wheel and turns it back towards the storm, and they hit a man. All three rush out of the car to the man on the ground, and Jane prays for him not to be dead.
The scene changes to medieval times, while the voice of Odin narrates that humanity used to have an unwavering faith in gods like the Asgardians. When the Frost Giants invaded Earth, the Asgardians, led by Odin, drove them back to their own realm of Jotunheim and stole the source of their power, the Casket of Ancient Winters, leading to a new era of peace and stability.
We see Odin marching down a corridor, telling this tale to his young sons, Loki and Thor. Thor insists that the Frost Giants should be exterminated, but Odin wisely says that a true king should never seek out a fight, but always be prepared for one. Odin then states that he loves both his sons, but only one can be king.
Several years later, and Loki and Thor are now full-grown men, and it is Thor's 'big day' - he is about to ascend to the throne. However, the proceedings are interrupted by three Frost Giants killing some Asgardian guards and breaking into the vault where the Casket is kept. Odin tells the Casket's guard, the Destroyer, to protect the Casket, and it emerges from the wall, disintegrates all three Giants and steps back.
Thor is angered by the Frost Giants' invasion of Asgard, and sees it as a declaration of war. Odin gravely forbids Thor to travel to Jotunheim, but Thor disobeys, taking Loki and his four friends (Volstagg, Hogun, Fandral and Sif) to the bifrost bridge, where the Gatekeeper, a powerful being named Heimdall, grants them access, stating that he would like to discover how the Frost Giants escaped his gaze.
The six warriors travel to Jotunheim, where Thor confronts the Giants' leader, Laufey. Laufey warns Thor that his actions will have dire consequences, but Thor, outraged at being mocked, slays hundreds of Frost Giants with his hammer Mjolnir until Laufey unleashes a monolithic beast. The beast chases the six back out of Jotunheim and cuts off access to the bifrost; Thor then slays it with his hammer. Still, Frost Giants continue to overpower the six until Odin descends from the bifrost; the Frost Giants retreat, but Laufey warns Odin that Thor's actions meant war.
Back on Asgard, Odin sends the four warriors away and confronts Loki and Thor. Thor openly admits his declaration of war, calling his father an old man and a fool. Tearfully, Odin strips Thor of his power and sends him to Earth, accompanied Mjolnir, which contains all of Thor's godly powers, and can only be lifted by the worthy. Thor arrives on Earth and is hit by the stormchasers. Jane prays for him not to be dead, and Thor gets up, demanding to know which realm he is in. Thor continues to bewilder the three humans until Darcy tazers him. They take Thor to a hospital, where he overpowers several doctors and nurses and is tranquilized. Jane takes Thor to an apartment and then out to a cafe, where Thor hears that some miles west of where they are is a hammer, buried in dirt, which no-one can lift. Knowing where to find Mjolnir, Thor takes his leave of Selvig, Jane and Darcy and sets off.
Jane, Selvig and Darcy arrive back at their equipment station to find that all their stuff, even Darcy's iPod, have been carted off by the S.H.I.E.L.D agency. Agent Phil Coulson greets them, and says that S.H.I.E.L.D need their equipment to research the storms. Coulson says that they are the good guys. Defeated, Jane decides to find Thor, and gives him a lift to the site of the hammer, which has now been cordoned off by the same agency.
Thor fights his way through hundreds of specially trained guards to find Mjolnir, but finds he cannot lift it, and therefore has no power. Realising that he has been decreed not worthy of the power, Thor sinks to his knees and allows himself to be handcuffed.
Back on Asgard, Loki has revealed to Thor's friends that he told the Gatekeeper to inform Odin that they were going to Jotunheim; meaning Loki saved their lives, but also caused Thor to be banished. Loki travels to the vault and looks in wonder at the Casket, then picks it up; he is interrupted by Odin. Loki turns around, revealing his skin is blue like a Frost Giants', and asks if he is cursed. Odin replies that on the day the took the Casket from Jotunheim, he also found Laufey's son, a baby boy. Odin changed the baby's skin to look like an Asgardian, and adopted him as his son - Loki. Angrily, Loki accuses the old man of favouring Thor, as a Frost Giant could never sit on the throne of Asgard. Suddenly Odin clutches his chest and falls down. Loki calls for help, and the guards put Odin into stasis, in hope that he will recover.
On Earth, Thor is being interrogated by Coulson, who asks where Thor received his training. Thor does not reply. Coulson leaves and suddenly Loki appears, saying that Odin has died and Thor's banishment is the only thing that keeps Asgard from war with Jotunheim - Thor can never return from exile. Thor is rescued by Selvig, who buys Thor a round of drinks in a pub and forces Thor to promise he will not hurt Jane, who clearly has developed romantic feelings for Thor. Thor promises, and the two return to Jane's 'temporary living accommodation', a trailer with basic living devices. Thor tells Jane that he really is the god of thunder, and that he doesn't know what to do.
On Jotunheim, Loki visits Laufey. Loki admits that he was the one who showed the Frost Giants the way into Asgard so they could try to steal the casket, as seen at the beginning of the movie. Loki offers Laufey a deal - Loki will sneak some Frost Giants, Laufey included, into Asgard, and they will kill Odin where he lies. Loki will then become king. In return, Loki will give the Casket to the Frost Giants and they can restore Jotunheim. Laufey accepts and the plan is put into motion.
Thor's four warrior friends, after discovering Loki's plans to become king, decide to travel to Earth to find Thor. They find him, and tell him that Odin is not really dead, and Thor realises the extent of his brother's madness. Loki freezes the Gatekeeper solid for his treachery, and sends the Destroyer to Earth to 'destroy everything'. Thor tells the warriors and the stormchasers to run, and sacrifices himself to make the Destroyer leave. As Jane cries over Thor's body, the hammer of Mjolnir begins to shake, then bursts free of the earth and flies to Thor - he has proved himself worthy. Life and godly powers restored, Thor defeats the Destroyer, and he and the four warriors set off for Asgard, Thor giving Jane a goodbye kiss and a promise of return.
The Gatekeeper breaks out of his encasing of ice and opens the bifrost, allowing Thor and his friends to return. In Odin's chamber, Laufey hopes that Odin can still see what is going on around him, so he will know who killed him; however, Loki then kills Laufey, and is decreed a hero for saving the king. Loki opens the bifrost permanently to destroy Jotunheim, hoping to prove himself worthy to Odin. Thor arrives and a fight ensues between the brothers. Loki, surprised, comments on how Thor, once an arrogant warrior, has now become a self-sacrificing man, and Thor replies that Loki is now insane. Thor uses his hammer to destroy the bifrost bridge and stop Jotunheim's destruction, fully aware that this means he will never see Jane again. Enraged, Loki tackles Thor off the edge of the bridge and into the void; however, they are saved by Odin, who has awoken. Loki screams that everything he did was for Odin, but Odin denies this, and Loki lets go and falls into darkness, while Odin tearfully helps Thor up.
Later, Thor apologizes to his father for his arrogance, and hopes one day he will make him proud. Odin replies that Thor already has. Thor admits he still has much to learn. On Earth, Jane and her team are searching for a way to open a portal to Asgard.
Erik Selvig has been taken to an underground S.H.I.E.L.D facility, where he is coldly geeted by Nick Fury. Fury shows Selvig a briefcase containing the Tesseract, which Fury describes as, 'power. If we can harness it, maybe unlimited power'. An invisible Loki appears in a nearby reflection and whispers 'well, I guess that's worth a look'. Selvig repeats Loki's words.
- Chris Hemsworth - Thor Odinson
- Tom Hiddleston - Loki Laufeyson
- Natalie Portman - Jane Foster
- Anthony Hopkins - Odin Borson
- Jaimie Alexander - Sif
- Colm Feore - Laufey
- Stellan Skarsgard - Erik Selvig
- Joshua Dallas - Fandral
- Ray Stevenson - Volstagg
- Tadanobu Asano - Hogun
- Idris Elba - Heimdall the Gatekeeper
- Kat Dennings - Darcy Lewis
- Rene Russo - Frigga
- Clark Gregg - Phil Coulson
- Maximiliano Hernandez - Jasper Sitwell
- Samuel L. Jackson - Nick Fury
- Jeremy Renner - Clint Barton
- Erik Selvig references Bruce Banner's gamma incident.
- Clint Barton appears as an agent of S.H.I.E.L.D.
- When The Destroyer arrives on Earth Jasper Sitwell asks "Is that one of Stark's?"
- Erik Selvig has a conversation with Nick Fury.
- Nick Fury shows Selvig a metal case containing The Tesseract.