As Frodo and Sam leave, a band of orcs captures Merry and Pippin and kills Boromir. Aragorn, Legolas, and Gimli decide to let the Ringbearer go and instead rescue Merry and Pippin. Aragorn, Legolas, and Gimli pursue the orcs as they run across the fields of Rohan toward Saruman's fortress at Isengard. They find evidence that the orcs of Sauron and Saruman have quarreled and that either one hobbit is still alive, but they begin to lose hope as they fall farther and farther behind. After three days of running with little rest, they meet a troop of the riders of Rohan. The horsemen's leader, Éomer, informs the trio that the orcs were destroyed on the edge of Fangorn forest, with no survivors. He lends them horses to continue the pursuit, but offers little hope that the hobbits escaped the battle.
Although treated roughly by the orcs, Merry and Pippin remain unharmed. When the Rohirrim attack, the hobbits run to the safety of the forest. They eventually make their way through the dense woodland to a stony hill where they meet Treebeard the Ent. This ancient being looks like a walking and talking tree and is the guardian of the forest. He does not particularly care about Sauron, but he takes the threat of Saruman very seriously. The wizard once cared for the forest and learned much from Treebeard, but now he chops down the trees to fuel the war machines of Isengard. Treebeard calls an Entmoot, a meeting of many Ents who decide to fight Saruman.
Aragorn, Legolas, and Gimli find the remains of the orcs at the edge of Fangorn and follow the hobbits' tracks into the forest. They meet a white-robed wizard under the trees whom they at first take to be Saruman, but he reveals himself as Gandalf returned. When he fell in Moria he continued to fight the Balrog and eventually overcame him. The ordeal changed him, and he has returned in white to signify that he is now the head of the wizard's council. Gandalf takes the three remaining members of the Fellowship to Edoras, Rohan's capital, to counter the evil influence of Saruman. King Théoden has listened to the evil counsels of Wormtongue, Saruman's spy, succumbing to old age and despair. Gandalf rouses him from his stupor and reveals Wormtongue's treachery. Éowyn, Théoden's niece and Éomer's sister, admires Aragorn. Théoden then musters his army and leads them to the fortress of Helm's Deep. The Rohirrim defeat Saruman's enormous army, although they suffer many losses. Unexpectedly, a great forest of trees appears in the valley and destroys all the orcs that try to escape.
After the battle at Helm's Deep, Gandalf takes Théoden and his companions to Isengard. Where they had expected to find a wizard's stronghold, they find ruins. Merry and Pippin, left to wait at the gate, explain that the Ents have destroyed Isengard, although Saruman himself remains hidden in the impenetrable tower of Orthanc. Gandalf speaks to the fallen wizard, offering him a chance to repent. When Saruman refuses, Gandalf breaks his staff. Wormtongue, perhaps trying to hit Saruman, throws a crystal ball out the window. Later that night, Pippin sneaks a look into the stone and encounters Sauron himself. Fortunately, Gandalf breaks the contact before the hobbit has a chance to reveal Frodo's quest to destroy the Ring.
Meanwhile, Frodo and Sam have become lost among the pathless hills of the Emyn Muil. They realize that Gollum is following them and manage to capture the corrupt creature. Despite Sam's misgivings, Frodo bargains with Gollum for guidance to the Black Gate of Mordor. Gollum agrees and leads them out of the hills and through the Dead Marshes, a haunted swamp. When they reach the Black Gate, Frodo and Sam despair of passing the constant guard but resolve to make the attempt. Gollum, however, begs them to take another way — a secret passage that only he knows. Desperate, Frodo agrees, and they turn aside from the gate.
Traveling toward the pass of Cirith Ungol, the hobbits discover a surprisingly pleasant countryside. When they pause for a stewed rabbit dinner, however, they find themselves in the middle of a battle between an army journeying to Mordor and a company of Gondorian men. Gollum sneaks off, but the hobbits are captured by the men. The Gondorians lead them to a secret base, where they learn that the men's captain is Faramir, Boromir's younger brother. Faramir realizes their quest and their burden, but he does not attempt to take the Ring from Frodo. With Frodo's help, he does capture Gollum, an action that breaks the fragile trust that had begun to form between the two Ringbearers. Faramir advises the travelers against the path they have chosen, but he can suggest no alternative ways into Mordor.
After leaving Faramir, the hobbits make their way past Minas Morgul, the home of the Black Riders. They take a hidden path into the mountains behind the fortress, a path made of steep staircases and tunnels bored through the rock. At the top of the stairs, the hobbits find themselves in a reeking tunnel clogged with tough webs. This is the lair of Shelob, a giant evil spider with an insatiable appetite. Gollum hopes to recover the Ring from the remains of Frodo and Sam, after Shelob has eaten them. With the aid of Galadriel's gift, a phial containing the pure light of elven stars, they escape Shelob's tunnel, but she attacks again from another angle and stings Frodo. Sam drives her from Frodo's fallen body, but he finds his master cold and lifeless. Nearly incapacitated by grief, Sam forces himself take the Ring from Frodo and continue the quest.