Thor is a 2011 American superhero film based on comic book character of the same name published by Marvel Comics and is the fourth film released in the Marvel Cinematic Universe. The film was released on April 21, 2011 in Australia and is scheduled for release on May 6, 2011 in the United States. Thor was also released in 3-D and IMAX 3D. The film stars Chris Hemsworth, Natalie Portman, Tom Hiddleston, Anthony Hopkins, and Stellan Skarsgård with Kenneth Branagh directing a script by Ashley Edward Miller, Zack Stentz, and Don Payne.

The project of a film adaptation of Thor was in "development hell" for several years before Marvel Studios signed Protosevich to write its script in 2006. Matthew Vaughn was to direct the film in late 2008 for a release in 2010. Branagh replaced Vaughn in late 2008, and the film's release was rescheduled into 2011. The main characters were cast in 2009, and principal photography took place from January to May 2010.

In 970 A.D., Odin (Anthony Hopkins), king of Asgard, wages war against the Frost Giants of Jotunheim and their leader Laufey (Colm Feore), to prevent them conquering the Nine Realms, starting with Earth. The Asgardian warriors defeat the Frost Giants and seize the source of their power, the Casket of Ancient Winters.

In the present, Odin's son Thor (Chris Hemsworth) prepares to ascend to the throne of Asgard as King but is interrupted when the Frost Giants attempt to retrieve the Casket. Against Odin's order, Thor travels to Jotunheim to confront Laufey, accompanied by his brother Loki (Tom Hiddleston), childhood friend Sif (Jaimie Alexander) and the Warriors Three: Volstagg (Ray Stevenson), Fandral (Joshua Dallas) and Hogun (Tadanobu Asano). A battle ensues which forces Odin to intervene and save their lives, destroying the fragile truce between the two races. For his arrogance, Odin strips Thor of his godly power and exiles him to Earth, accompanied by his hammer - and source of his power - Mjolnir, now protected by a spell to allow only the worthy to wield it.

Thor arrives in New Mexico and is found by scientist Jane Foster (Natalie Portman), her assistant Darcy Lewis (Kat Dennings) and mentor Dr. Erik Selvig (Stellan Skarsgard) while Mjolnir is discovered by agent Phil Coulson (Clark Gregg) and confiscated by S.H.I.E.L.D. Coulson forcibly acquires Jane's data about the wormhole that delivered Thor to Earth. Thor, having discovered the location of Mjolnir, attacks the S.H.I.E.L.D facility to retrieve it, only to find himself unable to lift it, and is captured by S.H.I.E.L.D. He is eventually freed and resigns himself to exile on Earth as he develops a romance with Jane.

Meanwhile, Loki discovers that he is Laufey's son and was adopted by Odin after the war ended. When Odin, overcome with stress, falls into the Odinsleep - a prolonged period of deep sleep to recuperate - Loki becomes king and offers Laufey the chance to kill Odin as well as retrieve the Casket. Sif and the Warriors Three, unhappy with Loki's rule, attempt to discover a way to end Thor's exile, convincing Heimdall (Idris Elba), gatekeeper of the Bifröst Bridge - the means of travelling between worlds - to allow them passage to Earth. Aware of their plan, Loki sends the Destroyer, a seemingly-indestructible automaton, to pursue them and kill Thor. Sif and the Warriors Three find Thor but are attacked and defeated by the Destroyer, forcing Thor to sacrifice himself to save them and Jane. In doing so, he proves himself worthy to wield Mjolnir, and the hammer returns to him, restoring his powers and allowing him to defeat the Destroyer before he prepares to return to Asgard and confront Loki.

In Asgard, Loki betrays and kills Laufey, revealing his true plan to use their attempt on Odin's life as an excuse to destroy Jotunheim with the Bifröst Bridge and prove himself worthy to Odin. Thor arrives and fights his brother before destroying the Bifröst Bridge to stop his plan, stranding himself in Asgard. Odin awakens and prevents the brothers from falling into the abyss created in the wake of the Bifröst Bridge's destruction, but Loki, believing he has failed Odin, allows himself to fall to his apparent death. Thor makes amends with Odin, admitting he is not ready to be King, while on Earth Jane and her team search for a way to open a portal to Asgard so that Jane can reunite with Thor.

In a post-credits scene, Dr. Selvig is taken to a S.H.I.E.L.D. facility where Nick Fury (Samuel L. Jackson) asks him to study the Cosmic Cube, an item of untold power. Selvig is revealed to be under the control of Loki who is now on Earth.