Awakening from a dream of Gandalf fighting the Balrog in Moria, Frodo Baggins finds himself, along with Samwise Gamgee, lost in the Emyn Muil near Mordor. They discover that 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 Merry and Pippin 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. In Fangorn, Aragorn's group encounters Gandalf, who after his fight against the Balrog 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 the Hornburg at 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 Théodred's horse Brego and rides to Helm's Deep, witnessing Saruman's army marching toward 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 honour their alliance to men, and they dispatch a company 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, warning Théoden of Saruman's army approaching. Théoden prepares for battle despite being vastly outnumbered. A company of Lothlorien Elves arrives to aid the people of Rohan, shortly before Saruman's army attacks the fortress. The Uruk-hai breach the outer wall with explosives and during the ensuing charge, 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 trees. 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, younger 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, but Sam saves him and reminds a disheartened Frodo that they are fighting for the good still left in Middle-earth. Impressed by Frodo's resolve, Faramir releases them. Feeling betrayed, Gollum decides he will reclaim the Ring by leading Frodo to "her" upon arriving at Cirith Ungol.
The hobbit, Sméagol, is fishing with his cousin Déagol, who discovers the One Ring in the river. Sméagol's mind is ensnared by the Ring, and he kills his cousin for it. Increasingly corrupted physically and mentally, he retreats into the Misty Mountains and becomes known as Gollum.
Centuries later, during the War of the Ring, Gandalf leads Aragorn, Legolas, Gimli, and King Théoden of Rohan to Isengard, where they reunite with Merry and Pippin. Gandalf retrieves Saruman's palantír, and the group returns to Edoras to celebrate their victory at Helm's Deep.[a] Pippin looks into the palantír, seeing Sauron and a burning tree. Gandalf deduces that the enemy plans to attack Gondor's capital Minas Tirith; he rides there to warn Gondor's corrupt steward Denethor. Pippin, who accompanies him, swears fealty to Denethor, whose now-dead heir Boromir had saved his life;[b] on Gandalf's instruction, Pippin triggers the lighting of the beacons, which call for help from Rohan.
Frodo, who carries the Ring, and Sam continue their journey towards Mordor, unaware that Gollum, now their guide, plans to betray them and take the Ring for himself. The trio witness the Witch-king of Angmar, lord of the nine Nazgûl, setting off towards Gondor with his army of Orcs. Gollum conspires to frame Sam for eating food supplies and desiring the Ring; influenced by the growing power of the Ring, Frodo is taken in by the deception, and orders Sam to go home. Gollum then tricks Frodo into venturing into the lair of the giant spider Shelob. Frodo narrowly escapes and confronts Gollum, who falls down a chasm after a scuffle. Shelob discovers, paralyses, and binds Frodo, but is wounded and driven away by a returning Sam, who, mourning Frodo's apparent death, takes the Ring. Sam realises his mistake when a group of Orcs takes Frodo captive but manages to rescue Frodo as the Orcs fight among themselves. Now inside Mordor, the hobbits continue towards Mount Doom, their destination.
As King Théoden gathers his army, Elrond tells Aragorn that Arwen is dying, having refused to leave Middle-earth. Elrond gives Aragorn Andúril, reforged from the shards of King Elendil's sword Narsil, and urges him to commit to claiming Gondor's throne, to which he is the heir. Joined by Legolas and Gimli, Aragorn travels the Paths of the Dead, and pledges to release the ghosts there from their curse should they come to Gondor's aid. Meanwhile, Faramir, who was earlier overwhelmed and driven back to Minas Tirith by the Witch-king, is gravely wounded in a suicide charge; believing his son to be dead, Denethor falls into madness. Gandalf marshals the defenders, but the huge Orc army breaks into the city. Denethor attempts to burn himself and Faramir on a pyre, but Pippin alerts Gandalf and they rescue Faramir. Denethor, set ablaze and in agony, jumps to his death.
Théoden arrives and leads his army against the Orcs. Despite initial success against Orcs in the ensuing battle, they are decimated by the Oliphaunt-riding Haradrim and the Witch-king mortally wounds Théoden; however, his niece Éowyn slays the Witch-king with Merry's help. Théoden dies in his niece's arms. Aragorn then arrives with his Army of the Dead, who overcome Sauron's forces. Their oath is fulfilled, and the Dead are released from their curse. Aragorn decides to march on Mordor to distract Sauron from Frodo, now extremely weak, and Sam; all of Sauron's remaining forces march to meet Aragorn's diversion, allowing the hobbits to reach Mount Doom. Gollum, who survived his earlier fall, attacks them, but Frodo still manages to enter the mountain. There, he succumbs to the Ring's power, putting it on his finger, but Gollum manages to bite off his finger and reclaim it. They struggle together and stumble off the ledge. Frodo clings to it with one hand as remorse and guilt flood his mind in the wake of his succumbing to the Ring when Sam's unwavering faith in his friend demands Frodo reach for Sam's stretched hand, saving Frodo's life. Gollum falls and dies; the Ring, which fell with him, disintegrates in the lava, causing Barad-dûr to crumble as the Eye of Sauron explodes, destroying the Dark Lord once and for all. Aragorn's army emerges victorious as its enemies and the lands of Mordor collapse into the earth, and Mount Doom erupts, with Frodo and Sam narrowly escaping the lava.
Gandalf rescues the hobbits with the help of eagles, and the surviving Fellowship is happily reunited in Minas Tirith. Aragorn is crowned King of Gondor and marries Arwen. The Hobbits return home to the Shire, where Sam marries Rosie Cotton. Four years later, Frodo, still plagued by the pain of his wound inflicted by the Witch-king and suffering from trauma, departs Middle-earth for the Undying Lands with his uncle Bilbo, Gandalf, and the Elves. He leaves Sam the Red Book of Westmarch, which details their adventures. Sam returns to the Shire, where he embraces Rosie and their children.