Two Hobbits, Sméagol and Déagol, are fishing when Déagol discovers the One Ring in the river. Sméagol is ensnared by the Ring, and kills his friend for it. He retreats into the Misty Mountains as the Ring twists his body and mind, until he becomes the creature Gollum.
Centuries later, during the War of the Ring, Gandalf leads Aragorn, Legolas, Gimli, and King Théoden to Isengard, where they reunite with Merry and Pippin. Gandalf retrieves the defeated Saruman's palantír. Pippin later looks into the seeing-stone and is seen by Sauron. From Pippin's description of his visions, Gandalf surmises that Sauron will attack Gondor's capital Minas Tirith. He rides there to warn Gondor's steward Denethor, taking Pippin with him.
Gollum leads Frodo Baggins and Samwise Gamgee to Minas Morgul, where they watch the Witch-king of Angmar, leader of the nine Nazgûl, lead an army of Orcs towards Gondor. The hobbits begin climbing a stair carved in the cliff face that leads to a secret tunnel into Mordor, unaware that Gollum plans to kill them and take the Ring. The Witch-king and his forces strike and overwhelm Osgiliath, forcing Faramir and his garrison to retreat to Minas Tirith.
Gollum disposes of the Hobbits' food, blaming Sam. Believing that Sam desires the Ring, Frodo tells him to go home before he and Gollum continue to the tunnel leading to Mordor. Gollum tricks him into venturing into the lair of the giant spider Shelob. Frodo narrowly escapes and confronts Gollum, telling him that he must destroy the Ring for both their sakes. Gollum attacks Frodo but falls down a chasm. Frodo continues on, but Shelob discovers, paralyses, and binds him. Sam returns and injures Shelob, driving her away, but then hides as Orcs appear and take Frodo with them. The Orcs then fight over ownership of Frodo's mithril vest, allowing Sam to escape with Frodo and continue their journey.
As King Théoden gathers the Rohirrim army, Aragorn learns from Elrond that Arwen is dying, having refused to leave Middle Earth to be with Aragorn after seeing a vision of their future son. Arwen convinced a reluctant Elrond to order the shards of King Elendil's sword, Narsil, be reforged into Andúril so that Aragorn can reclaim his birthright and gain reinforcements from the ghostly Dead Men of Dunharrow. Joined by Legolas and Gimli, Aragorn travels to the Dead Men's lair, pledging to release them from Isildur's undead curse should they come to Gondor's aid.