Frodo Baggins and Sam Gangee journey to Mordor to cast the One Ring into Mount Doom. During their quest, they are attacked by Gollum, the ring's former owner, but Frodo overpowers him and requests that Gollum lead them to Mordor.
Aragorn, Legolas, and Gimli pursue the Uruk-hai, who had kidnapped the Hobbits Merry Brandybuck and Pippin Took. The Ururk-hai are massacred by the Rohirrim army of Rohan while Merry and Pippin meet Ent Treebeard in Fangorn Forest. Aragorn's group meet the Rohirrim and learn they were banished by their king, Theoden, under the manipulation of Gríma Wormtongue, the servant of Saruman. In Fangorn Forest, Aragorn's group meet Gandalf, who was revived to help save Middle Earth.
Gollum leads Frodo and Sam through the Dead Marshes and evades the Nazgul. Upon discovering that the Black Gate to Mordor is closed, Gollum searches for another entrance, and finds himself becoming loyal to Frodo. The three are captured by Faramir, the brother of Boromir, who had been killed by the Uruk-hai. Faramir intends to take Frodo to Gondor upon learning he has the Ring, and Frodo exposes Gollum, leading to his capture.
Gandalf and Aragorn's group arrive at Rohan's capital Edoras, where Gandalf releases Theoden from Saruman's control and exiles Wormtongue. Learning of Saruman's plan to attack Rohan with an army of Uruk-hai, Theoden opts to relocate his citizens to Helm's Deep. Gandalf departs to find the Rohirrim, led by Theoden's nephew, Eomer. Meanwhile, Theoden's niece Eowyn becomes smitten by Aragorn. Aragorn plummets into a river during a Warg attack and is presumed dead, but he manages to arrive at Helm's Deep.
In Fangorn, Merry and Pippin convince the Ents to participate in the war by showing them the destruction unleashed by Saruman around Isengard. The Ents arrive at Isengard and capture Saruman. The Uruk-hai arrive at Helm's Deep, and a battle ensues. Theoden begins to lose hope, but is reinvigorated by Aragorn. Gandalf arrives with the Rohirrim, and they overpower and defeat the Uruk-hai. Meanwhile, Frodo, Sam, and Gollum are attacked by the Nazgul at the fallen Gondor city Osgiliath. Frodo begins to become entranced by the Ring and is nearly captured by the Nazgul, but Sam knocks him down a flight of stairs. Frodo nearly kills Sam, but relents upon recognizing him.
Faramir releases Frodo, Sam and Gollum, allowing them to continue to Mordor. However, Gollum becomes disillusioned with Frodo as a result of his betrayal, and schemes to retake the Ring by luring Frodo and Sam towards a creature referred to as "her".
- Elijah Wood as Frodo Baggins.
- Ian McKellen as Gandalf the White.
- Viggo Mortensen as Aragorn.
- Liv Tyler as Arwen Evenstar.
- Orlando Bloom as Legolas.
- John Rhys-Davies as Gimli/Treebeard.
- Christopher Lee as Saruman.
- Sean Astin as Sam Gamgee.
- Billy Boyd as Pippin Took.
- Dominic Monaghan as Merry Brandybuck.
- Andy Serkis as Gollum.
- Hugo Weaving as Elrond.
- Craig Parker as Haldir.
- Bernard Hill as Theoden.
- Brad Dourif as Gríma Wormtongue.
- Miranda Otto as Eowyn.
- David Wenham as Faramir.
- Karl Urban as Eomer.