The film begins with a flashback set to the first film, with Gandalf battling the Balrog on the Bridge of Khazad-dûm, but this time continues from Gandalf's perspective, with the scene continuing to follow both as they hurtle down below, fighting while in free-fall. Frodo awakens from his dream and continues his journey with his trusted and loyal friend, Sam. They are then attacked by the ring-possessed Gollum wishing to retrieve "his precious" from the ones he thinks stole it from him. The Hobbits subdue and bind him with Sam's Elven rope given to him by the Elven elder Galadriel in Lórien. Sam distrusts Gollum and wishes to abandon him, but Frodo understands the burden of the creature and takes pity on him. Realizing they are lost in the Emyn Muil and in need of a guide, Frodo persuades Gollum to lead them to the Black Gate of Mordor.

In Rohan, the pack of Uruk-hai run across the grassy landscape with their captives Merry and Pippin. Aragorn, Legolas and Gimli are in pursuit, following three days of running, Legolas surmises the Hobbits are being taken to Isengard, where Saruman is marshalling his Uruk-hai forces to do the bidding of Sauron. In the kingdom of Rohan, home of the horse lords, King Théoden is mentally and physically weak due to the enchantments of his steward, Gríma Wormtongue, who is secretly in the service of Saruman. Orcs and Wild Men of Dunland incited by Saruman freely roam the land and kill the people including the king's only son Théodred. Théoden's nephew Éomer interrogates Gríma, angrily realizing he has lustful eyes for Éomer's sister Éowyn and that he is now an agent of Saruman. Gríma banishes Éomer for undermining his authority and Éomer sets forth to gather the remaining loyal men of the Rohirrim throughout the land.

Frodo and Sam traverse the Dead Marshes, passing the undead fallen warriors of the Second Age who haunt the marshes and evading a newly seated Ringwraith on his flying fell beast. Later they reach the Black Gate, finding it to be heavily guarded, (they observe a contingent of Easterlings from Rhûn arrive to reinforce the garrison) only to have Gollum reveal to them a less risky path: Sam remains distrustful, but Frodo gives him the benefit of the doubt. Meanwhile, Éomer and his Rohirrim ambush and kill all of the Orcs and Uruk-hai holding the two Hobbits captive at nightfall. During the battle, Merry and Pippin narrowly escape their captors by fleeing into the trees where they are aided by Treebeard the oldest of the Ents.

Éomer later encounters Aragorn, Legolas and Gimli and in turn tells Aragorn there were no survivors of the Orc/Uruk-hai slaughter. Upon arriving at the battle site, Aragorn uses his tracking skills and finds hobbit tracks that lead into nearby Fangorn forest. The three discover a wizard who is ultimately Gandalf reborn, now known as Gandalf the White. The quartet proceed to travel to Edoras, where they exorcise Saruman's hold on King Théoden and banish Wormtongue. Théoden is co