As Frodo Baggins, Sam Gangee, and Gollum journey through Mordor, the remaining Fellowship, consisting of Gandalf, Aragorn, Legolas, Gimli, joined by Theoden and Eomer, meet the Hobbits Merry and Pippin in the forest of Fangorn. They meet the Ent Treebeard at the tower of Orthanc, where the wizard Saruman is trapped. Gandalf, knowing that Saruman's poses no threat anymore, decides to spare him. Pippin becomes entranced by a palantir, but Gandalf hides it.
Sam overhears that Gollum intends to lure Frodo into the clutches of the giant spider Shelob, but Frodo insists that Gollum is their guide. Meanwhile, Pippin looks into the palantir and is subjected to mental torture by Sauron. Gandalf, deducing that Sauron intends to attack the city of Minas Tirith, rides off with Pippin to warn them. Meanwhile, Arwen insists to Elrond that she cannot leave Aragorn, having foreseen that she will have a son with him.
Gandalf and Pippin arrive at Minas Tirith, where the steward Denethor is mourning over the death of his son, Boromir. Pippin, to compensate for Boromir's demise, pledges loyalty to him. As the Witch-king of Angmar leads his army of Orcs to attack the city, Aragorn, Theoden, and the Rohirrim prepare for war. Theoden's daughter, Eowyn, joins the army, disguised as a soldier. The Orcs capture Osgiliath, forcing Denethor's other son, Faramir, to retreat. Gandalf rescues him, and Faramir convinces Gandalf of Gollum's intentions to betray Frodo and Sam. Denethor, displeased with Faramir, sends him on a suicide mission to reclaim the city. Faramir is defeated and put into a coma and returned to the city. Meanwhile, Gollum blames Sam for eating their food provisions, and Frodo demands that Sam return home.
Elrond presents to Aragorn the reforged sword of Narsil, renamed Anduril, urging him to use it to summon the Dead Men of Dunharrow. Meanwhile, as the Orcs lay siege to Minas Tirith, Denethor goes into a maddened state, believing Faramir to be dead. Gandalf assumes control of the defenses as the Witch-king and the Nazgul attack the city. Meanwhile, Gollum lures Frodo to Shelob, but he manages to escape. However, he is paralysed by Shelob's stinger. Sam, believing Frodo to be dead, takes the One Ring. However, as the Orcs take Frodo, Sam overhears that he is alive. Meanwhile, Gandalf and Pippin save Faramir from being burned with Denethor, but Denethor kills himself. The Witch-king murders Theoden before he himself is killed by Eowyn. Aragorn, aided by the Dead Men of Dunharrow, drives off the Orc army. Their oath fulfilled, the Dead Men are released of their curse and they are allowed to rest in peace.
Sam reunites with Frodo and the two journey to Mount Doom to destroy the Ring. The Fellowship and the Rohirrim arrive to battle the Orc army as a distraction. In the midst of the battle, Frodo and Sam are attacked by Gollum, but Frodo manages to arrive in Mount Doom. However, he gives into the Ring's control and refuses to destroy it. Gollum subsequently bites off Frodo's finger to take the Ring, but Frodo attacks Gollum. The scuffle results in Gollum, along with the Ring, plummeting into the pits of Mount Doom, destroying both of them. With the Ring destroyed, Sauron begins to fade away. Mount Doom is destroyed, and the Ringwraiths and the Nazgul are killed, along with Sauron's army.
Gandalf rescues Frodo and Sam and they awaken in Minas Tirith. Aragorn is crowned king of Minas Tirith and marries Arwen. Frodo, still suffering from the wounds he received on the journey, decides to sail to Valinor with Gandalf, Bilbo, Elrond, and Galadriel. Frodo finishes his book and gives it to Sam, bidding his farewells to him, Merry, and Pippin, before setting off to Valinor.
- 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 Rhy-Davis as Gimli / Treebeard.
- Sean Astin as Sam Gangee.
- Billy Boyd as Pippin Took.
- Dominic Monaghan as Merry Brandybuck.
- Andy Serkis as Gollum.
- Hugo Weaving as Elrond.
- Ian Hlm as Bibo Baggins.
- Bernard Hill as Theoden.
- Miranda Otto as Eowyn.
- David Wenham as Faramir.
- Karl Urban as Eomer.
- John Noble as Denethor.
- Christopher Lee as Saruman (Extended Edition)
- Brad Dourif as Gríma Wormtongue (Extended Edition)
- Bruce Spence as The Mouth of Saron (Extended Edition)