The film begins in the desert where Jane Foster, her assistant Darcy and her mentor Erik Selvig are tracking strange weather patterns. Nothing has appeared so far and the others want to head back but Jane is convinced that her readings are right and something will appear here. True enough a storm suddenly picks up seemingly out of nowhere and they chase after it to take readings. While they chase after it Jane hits something that tumbles out of the storm which then disapeares. Getting out of the van the group find a man sprawled on the ground. As they check him over, the three wonder where he came from.
The film then flashes back to 970 AD where a Norweigean villiage comes under attack by the Frost Giants.They use a chest called the Casket of Antient Winters to freeze the area; killing the villagers. Odin narrates that in the past mankind was well aware they were not alone in the universe and that Earth was the Frost Giant's first target in an attempt to take control of the nine worlds. In the flashback Odin and his armies arrive on the scene and engage the Frost Giants. Odin explains that Asguard would not stand by and watch and after a long war they eventualy defeated the Giants and took the Casket back to Asguard for safe keeping.
On Asgard Odin shows his two young sons, Thor and Loki, the Casket in the castle vault. Thor is a bold child while Loki is more reserved. Thor proclaims he will one day hunt the Frost Giants like his father did. Odin tells the two that a good leader never goes looking for a fight but should always be ready for one and that while only one of them may sucseed him they both have great potential.
Years pass and Thor; now a great but arrogant and proud warrior is greated by the people of Asgard has he, about to become the King of Asgard enters the throne room to take his place as the new king of Asgard. While his family are proud of him Thor's cocky attitude worries Odin and Loki. While the corination begins several Frost Giants sneek into the vault and kill the guards. Odin senses what they are attempting moments before he pasess the crown to Thor and summons the Destroyer. The Destroyer; a giant mechanical Golem, emerges from one of the vault walls and kills the intruders with a powerful energy beam.
After the casket is put back on its pedestal Thor wants to go the Frost Gaint realm to seek retribution for the attack. Odin forbids it since he has a truce with Laufey; the Frost Giant king and he will not allow another war to happen because of the actions of a few.He also reminds his son that he is still king since the ceremony never finished leaving Thor to vent his frustration in the dining hall. Loki and Thor's friends Sif and the Warriors three ; Volstagg, Fandral and Hogun attempt to cheer him up.
Thor suddenly gets an idea telling the others he is going to Jotenheim, the Frost Giant realm, just to look for information and not a fight. The others think this is a crazy idea, one because its against Odin's orders and two the Frost Giant world is just as dangerous as its people. Thor eventualy gets them to come around and they head off to the Bifrost bridge. The only guard there is Heimdall, a powerfull all seeing Asguardian whose task is to guard the bridge from invasion. Loki tries to bluff for the group to get past but Heimdall knows what they are up to and allowes them to pass because he is currious to know how the Frost Giants slipped past him. The group prepares has the guardian opens the Bifrost portal; he warns them that should anything happen while they are gone that could be a threat to Asguard he wont reopen the portal when they call.
The warriors arive on Jotinheim through the Bifrost portal. The world is a frozen wasteland and there something forboding about the place wich everyone except Thor notices. They make it to the castle courtyard before they are met by Laufey and several Frost Giant guards. It quickly becomes apparent that the Giants have the group outnumbered but Laufey who is well aware of the truce offers them a chance to leave without blood shed. Thor wants answers and is ready to beat them out of someone but Loki convinces him that leaving is the best idea and he reluctently agrees.
Unfortunately one of the giants bates Thor with an insult, starting a fight between the Asguardians and the Giants. At first Thor and the others can hold their own useing their skills to defeat the giants, but Thor who is getting more engrossed in the battle calls Laufey to send out more foes, the others start to struggle. Sif is almost thrown off a cliff, Volstagg is burnt by the icy touch of one of the giants and warns the others to avoid physical contact with the giants. In the middle of the fight one of the giants manages to grab Loki's arm but instead of burning it it causes the skin on his arm to take on the appearance of a Frost giant's before he gets out of the creature's grip and his arm returns to normal.
When Fandral is skewered by an ice spear, Sif orders a retreat wich Thor doesn't notice and countinues the fight while the others flee. Laufey summons a monster to chase after them which corners them on the edge of the realm since Heimdall wont open the portal because of the risk. Thor quickly arrives to kill the monster only for the group to be surrounded by thousands of giants. When all seams lost Odin appeares. Thor wants his father to fight with them but Odin tried to negotiate with Laufey saying this battle was just the act of a few "Children" but Laufey says that Thor's actions just broke the truce and prepares to atack only for Odin to teleport everyone back to Asguard.
On Asguard Odin sends Sif and the warriors away for healing and begins to berate Thor asking if he has any idea what he has done. Thor says he father has grown soft and insults him. Odin admits Thor was right about him being foolish; foolish to think Thor was ready to be king. He grabss Thor's shoulder pad ripping of the clan crest and proceeds to banish him from Asguard for breaking the truce and re starting the war. He then kicks Thor through Bifrost before throwing his hammer after him, but not before putting a charm on it so only the worthy will be able to wield its power.
The film then catches up with the start with Jane checking over Thor has he comes around. Quickly realising he is on Earth he starts calling out to Odin to reopen the gate but nothing happens. The humans think he is nuts or drunk and Darcy tazers him unconsious when he starts acting violent. Not sure what to do they decide to take him to the hospital. As the group drives away, a shooting star (actually Thor's hammer, Mjolnir) land nearby.