Awakening from a dream of Gandalf fighting the Balrog in Moria, Frodo Baggins and Samwise Gamgee find themselves lost in the Emyn Muil near Mordor and discover they are being tracked by Gollum, a former bearer of the One Ring. Capturing Gollum, Frodo takes pity and allows him to guide them, reminding Sam that they will need Gollum's help to infiltrate Mordor.
Aragorn, Legolas, and Gimli pursue a band of Uruk-hai to save their companions Merry and Pippin, entering the kingdom of Rohan. The Uruk-hai are ambushed by a group of Rohirrim, allowing the Hobbits to escape into Fangorn Forest. Meeting Aragorn's group, the Rohirrim's leader Éomer explains that he and his men have been exiled by Rohan's king, Théoden, who is under the control of Saruman and his servant Gríma Wormtongue. Éomer believes Merry and Pippin were killed during the raid but leaves the group two horses. Searching for the Hobbits in Fangorn, Aragorn's group encounters Gandalf, who reveals that after his fight against the Balrog he was resurrected as Gandalf the White to help save Middle-earth.
Gandalf leads the trio to Rohan's capital, Edoras, where Gandalf frees Théoden from Saruman's control. Aragorn stops Théoden from executing Wormtongue, who flees. Learning of Saruman's plans to destroy Rohan with his Uruk-hai army, Théoden evacuates his citizens to the fortress of Helm's Deep. Gandalf departs to find Éomer and his followers, hoping they will fight for their restored king. Aragorn befriends Théoden's niece, Éowyn, who becomes infatuated with him. When the refugees travelling to Helm's Deep are attacked by Saruman's Warg-riding Orcs, Aragorn falls from a cliff and is presumed dead. He is found by his horse Brego and rides to Helm's Deep, witnessing Saruman's army marching to the fortress.
In Rivendell, Arwen is told by her father Elrond that Aragorn will not return. He reminds her that if she remains in Middle-earth, she will outlive Aragorn by thousands of years, and she reluctantly departs for Valinor. Elrond is contacted by Galadriel of Lothlórien, who convinces him that the Elves should honor their alliance to men, and they dispatch an army of Elves to Helm's Deep.
In Fangorn, Merry and Pippin meet Treebeard, an Ent. Convincing Treebeard that they are allies, they are brought to an Ent Council, where the Ents decide not to take part in the coming war. Pippin asks Treebeard to take them in the direction of Isengard, where they witness the deforestation caused by Saruman's war effort. Enraged, Treebeard and the Ents storm Isengard, trapping Saruman in his tower.
Aragorn arrives at Helm's Deep and reveals that Saruman's army is close and Théoden must prepare for battle despite being vastly outnumbered. The army of Elves from Lothlórien arrives, as does Saruman's army, and a battle ensues. The Uruk-hai breach the outer wall with explosives and kill the Elves' commander, Haldir. The defenders retreat into the keep, where Aragorn convinces Théoden to meet the Uruk-hai in one last charge. At dawn, as the defenders are overwhelmed, Gandalf and Éomer arrive with the Rohirrim, turning the tide of the battle. The surviving Uruk-hai flee into Fangorn Forest and are killed by the Ents. Gandalf warns that Sauron will retaliate.
Gollum leads Frodo and Sam through the Dead Marshes to the Black Gate, but recommends they enter Mordor by another route. Frodo and Sam are captured by Rangers of Ithilien led by Faramir, brother of the late Boromir. Frodo helps Faramir catch Gollum to save him from being killed by the Rangers. Learning of the One Ring, Faramir takes his captives to Gondor to bring the ring to his father Denethor. Passing through the besieged city of Osgiliath, Frodo tries to explain to Faramir the true nature of the ring, and Sam explains that Boromir was driven mad by its power. A Nazgûl nearly captures Frodo, who falls under the ring's power and momentarily attacks Sam, who reminds him that they are fighting for the good still left in Middle-earth. Impressed by Frodo's resolve, Faramir releases them. Gollum decides to betray Frodo and reclaim the Ring by leading the group to "Her" upon arriving at Cirith Ungol.
Elijah Wood as Frodo Baggins: A young hobbit sent on a quest to destroy the One Ring, the burden of which is becoming heavier.
Ian McKellen as Gandalf the White: An Istari wizard who fell fighting a Balrog and has now returned, more powerful than ever, to finish his task.
Viggo Mortensen as Aragorn Elessar: The heir-in-exile to Gondor's throne who has come to Rohan's defence.
Liv Tyler as Arwen Undómiel: An elven princess of Rivendell and Aragorn's true love.
Sean Astin as Samwise Gamgee: Better known as Sam, Frodo's loyal hobbit gardener and companion.
Cate Blanchett as Galadriel: The Elven-Queen of Lothlórien, who discusses Middle-earth's future with Elrond.
John Rhys-Davies as Gimli: A dwarf warrior and one of Aragorn's companions.
Also voices Treebeard: The leader of the ents, who is roused to anger after seeing that Saruman had decimated a large part of Fangorn Forest.
Bernard Hill as Théoden: The King of Rohan, who is under Saruman's spell until Gandalf heals him so he can lead his people once more.
Christopher Lee as Saruman the White: An Istari wizard waging war upon Rohan and devastating Fangorn Forest, who allied himself with Sauron in the previous film.
Billy Boyd as Peregrin Took: Better known as Pippin, a hobbit mistakenly captured by the Uruk-hai.
Dominic Monaghan as Meriadoc Brandybuck: Better known as Merry, a distant cousin of Frodo's who is mistakenly captured along with Pippin by the Uruk-hai.
Orlando Bloom as Legolas Greenleaf: An elven archer and one of Aragorn's companions.
Hugo Weaving as Elrond: The Elven-Lord of Rivendell who expresses doubt over his daughter's love for Aragorn.
Miranda Otto as Éowyn: Théoden's niece, who is in love with Aragorn.
David Wenham as Faramir: A prince of the Stewards of Gondor and captain of the Ithilien Rangers, who captures Frodo, Sam and Gollum.
Brad Dourif as Gríma Wormtongue: An agent of Saruman at Edoras, who renders Théoden incapable of decisions, and desires Éowyn.
Karl Urban as Éomer: Théoden's nephew and previous Chief Marshal of the Riddermark who was exiled by Gríma.
Sean Bean as Boromir: Faramir's older brother and a fallen member of the Fellowship who appears in flashbacks since his death, more prominently in the film's extended edition.
Andy Serkis as Gollum (voice/motion capture): A wretched hobbit-like[7] creature originally known as Sméagol Trahald, who owned the Ring for five centuries and now guides Frodo on his quest.
Craig Parker as Haldir: The leader of the Lórien Elves sent by Elrond and Galadriel to defend Helm's Deep.
John Leigh as Háma: The loyal doorwarden of the Golden Hall and a majordomo of Théoden.
Bruce Hopkins as Gamling: Théoden's chief lieutenant and a skilled member of the Royal Guard of Rohan.
John Bach as Madril: Faramir's closest aide, who informs him of battle preparations.
Nathaniel Lees as Ugluk: The leader of the band of Uruk-hai who kidnapped Merry and Pippin.
based on the second volume of J. R. R. Tolkien's The Lord of the Rings. The film is the second instalment in The Lord of the Rings trilogy, preceded by The Fellowship of the Ring (2001) and followed by The Return of the King (2003).