The film begins with a flashback set to the first film, with Gandalf battling the Balrog on the Bridge of Khazad-dûm as both continuously to hurtle down below. 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 Middle-Earth 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. 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 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 fallen warriors of the Second Age. They hide in a bush from a newly seated Ringwraith on his flying fell beast. Later they reach the Black Gate, only to have Gollum reveal to them a less risky path. Meanwhile, Éomer and his Rohirrim ambush and kill all of the Orcs and Uruk-hai 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 find 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 confronted with his dead son and rather than risk open war, decides to flee to a large fortification which in times of trouble has saved the people of Rohan called Helm's Deep. Gandalf leaves to find Éomer and his Rohirrim, promising to return within five days, as a strong attraction draws Éowyn to Aragorn during the journey to Helm's Deep. Wormtongue flees to Orthanc and tells Saruman of Rohan breaking from their grip; Saruman then decides to destroy Rohan.
In Ithilien, Gollum battles his split personality in an attempt to befriend Frodo and Sam and ultimately banishes his "evil" half. The two hobbits are witness to an ambush of Southrons but are taken captive by soldiers of Gondor. Meanwhile, along the journey to Helm's Deep, the travelers are attacked by Saruman's Wargs. During the battle, Aragorn is dragged by a Warg and falls off a cliff into a raging river as the grief-stricken survivors reluctantly move on to Helm's Deep. In Rivendell, Elrond knows that the age of Elves is ending and convinces Arwen that it is hopeless to stay and should leave for the Grey Havens. Elsewhere, Frodo and Sam are taken to Henneth Annûn and brought before Faramir, the younger brother of Boromir. Gollum eluded capture and in order to save his life, is lured into a trap unknowingly by Frodo. Faramir learns of the One Ring and to prove his worth to his father, decides the Ring shall go to Gondor. In Rohan, Aragorn washes up on the river's edge and is nudged back to consciousness by his horse, Brego. Battered but undaunted, he rides to Helm's Deep, passing Saruman's army of Uruk-hai; at least 10,000 strong. His arrival is met with relief but is short lived with the news of only 300 men in the stronghold. In the midst of despair, a battalion of Elves from Lórien, led by the Elf Haldir, arrives to assist in the ensuing battle. At Fangorn forest, Merry, Pippin, Treebeard and other Ents hold a Council to decide on the roles of the Ents in the war with Saruman.
In the pouring rain, the battle of Helm's Deep begins with a flurry of arrows from both human and Elven archers cutting down dozens of Uruk-hai. Scaling ladders are placed upon the Deeping Wall, and the Uruks swarm up to engage the defenders. The defenses are slowly being breached and the enemy manages to destroy the wall through its sewer drain, using a rudimentary explosive device created by Saruman. Despite Aragorn and Gimli's best efforts, the Uruk-hai manage to penetrate the main door and soon the stronghold is overrun. In the midst of battle, Haldir is slain and the few remaining Elves fall back into the Keep. In the Hornburg, however, the Uruks have also scaled the walls, and have breached the gate, forcing the defenders to retreat into the Keep. In Fangorn, Treebeard and the other Ents have decided to not have any involvement in the war. Frustrated, Pippin cleverly takes him to the section of Fangorn Forest Saruman has decimated near Isengard. Treebeard is filled with rage at Saruman's betrayal and commands all other Ents to seek vengeance. The Ents gather and embark upon 'the Last March of the Ents'.
Meanwhile, as the Keep is now under attack and realizing Gandalf's words before he departed, Aragorn and the rest make one last gallant ride on horseback to attack the Uruk-hai army, in a desperate bid to allow the Rohirrim's women and children to escape. As all hope seemed lost, Gandalf, Éomer, and two thousand Riders of the Rohirrim arrive to push back the Uruk-hai into Fangorn Forest, where the Ents and their Huorn allies are waiting to deal out death and destruction in revenge. Elsewhere, the Ents also attack Isengard, tossing stone and rock while collapsing a dam to flood its surroundings. At the ruins of Osgiliath, Faramir and the Hobbits are confronted by a Ringwraith and its fellbeast. With the help of Sam, Frodo narrowly escapes the beast's capture. Sam narrates how the story must go on and how they should keep pressing forward as Faramir decides to free them to finish their quest. Gandalf and the others now know a full war is inevitable (as Sauron will surely seek retribution for the defeat of Saruman) and hope rests with Frodo and Sam, who have resumed their journey to Mordor with Gollum. Accompanying them once again and having felt betrayed after his subsequent mistreatment by Faramir's men, Gollum's darker nature returns and decides to reclaim the ring by leading Frodo and Sam to "her".
