Aragorn follows Frodo's trail to the summit of Amon Hen; there he hears the sound of Boromir's horn amid the cries of many Orcs, and he runs down to help Boromir. He comes too late, however: Boromir is already dying, and in his last words he tells Aragorn about his attack on Frodo and about the attack of the Orcs, which have taken the Hobbits as prisoners. Gimli and Legolas return soon afterwards, and together they carry Boromir's body into a boat and let it float down the River. They also notice that a boat and Sam's baggage are missing, and conclude that Frodo and Sam must have crossed the River and headed towards Mordor while everybody else was looking for Frodo. Thus it seems unlikely that the remaining three companions could still find them, and Aragorn decides that they will pursuit the Orcs and attempt to rescue Merry and Pippin. They start the chase at once and with greatest speed, for the Orcs have already gained an advantage of several hours.
Aragorn, Gimli and Legolas continue their chase for three days, running with remarkable speed through the land of Rohan, but to their dismay it seems that Orcs are hardly resting at all, and their advantage is ever increasing. On the fourth day they meet a company of the Rohirrim, the Men of Rohan, led by Éomer, the Third Marshal of Rohan and the nephew of Théoden the King of Rohan. Aragorn explains them the purpose of their hunt after the Orcs, and Éomer tells that the Rohirrim have attacked and destroyed that band of Orcs two days ago, yet found no hobbits among them. They exchange some news, and Éomer is impressed with Aragorn and the quick journey that he and his two companions have made in the past few days. He gives them leave to travel through Rohan, and gives them spare horses. Aragorn, Gimli and Legolas continue along the trail and reach the site of the battle near the eaves of the great forest of Fangorn that evening. They find no traces of the hobbits. That night an old man appears (and disappears quickly) near their camp, and all their horses flee; they suspect the man to be the evil wizard Saruman.
Meanwhile Pippin and Merry suffer greatly as captives of the Orcs. The Orcs are under orders not to kill neither search the captives; for a while, the Orcs carry them, but then they are forced to run on their own, and the Orcs give them a strange liquor that strengthens them for a while. The orc-band consists of different kinds of Orcs: small ones from the Misty Mountains, some Orcs from Mordor (led by one Grishnákh) and the large Uruk-hai from Isengard, led by a captain called Uglúk. Quarrels arise about where the prisoners should be taken, and Uglúk's will prevails and they turn towards Isengard. Near the Fangorn forest they are surrounded by a group of the Riders of Rohan in the evening. The Riders light fires and await the dawn before the final attack. In the night Grishnákh comes to the hobbits and hopes to find the Ring himself; he takes them and tries to escape, but is detected and killed by the Riders. The hobbits crawl into the forest, unnoticed in the darkness. At dawn the Riders attack the orc-camp and all the Orcs are slain in the battle.
Merry and Pippin continue their way into the forest, and soon meet Treebeard the Ent. Ents are strange, tall, very old creatures whose appearance resembles that of the trees. They talk about a great many things: the hobbits tell Treebeard about their journey, and he tells them about the Ents, their history, and the Forest of Fangorn. The Ents are threatened by Saruman, whose orcs are destroying the forest and felling trees. Treebeard feels it is high time that something be done about it, and he assembles an Entmoot, a gathering of the Ents, where they debate this issue. Since Ents are never hasty, the assembly lasts for two days and two nights, but in the end they decide to attack Isengard (the ring of rocks in the middle of which the Tower of Orthanc, the dwelling of Saruman, is set). Treebeard takes the hobbits along on his march, and a great many Ents join it on the way.
Aragorn, Legolas and Gimli search the site of the battle the following morning, and find a leaf of mallorn and some crumbs of lembas. With this evidence of the hobbits' presence they continue their search into the forest of Fangorn and reach the hill where the hobbits had met Treebeard. Then they meet an old beggar-man whom they believe to be Saruman at first, but then he turns out to be Gandalf, who has defeated the Balrog and has now returned stronger than ever, and is now wearing a white robe. Gandalf tells them some news, particularly that the hobbits met Treebeard and that the Ents are heading towards Isengard; he advises them to go to Rohan and help in the war that is preparing there. He calls his horse, Shadowfax, and with him come also Aragorn's and Legolas' horses, who have met him the previous night after having fled in panic. Gandalf and the three companions ride to Edoras, the court of Théoden the King of Rohan.
They go to Meduseld, the hall of king Théoden. They are not very welcome there at first, and are even required to leave their weapons outside before seeing the king. Théoden is under influence of his counsellor Gríma (also called Wormtongue) who has convinced him that Gandalf is always a sign of nearing trouble, and should not be welcome. Gandalf silences Wormtongue with a bolt of lightning, and takes the king outside, into the fresh air and the light of the day. Here Théoden realises that listening to Gríma's whispers he felt much older and weaker than he really was, and now he opens his heart to Gandalf's advice and issues orders that the Rohirrim should prepare to move westwards towards Isengard at once, while those not able to join the army should retreat into the refuges in the mountains. Wormtongue objects this, but Gandalf reveals him as Saruman's spy; Théoden gives him the choice of joining the war or leaving for ever, and Gríma rides away. Then the king gives gifts to the companions: he lets Gandalf have Shadowfax, and gives excellent armour to Aragorn, Legolas and Gimli. Finally the host rides off, and Éowyn, Éomer's sister, is left to govern the remainder of the people of Rohan in the king's absence.
Gandalf turns towards Isengard with greatest speed, while the rest of the host ride towards Helm's Deep, a stronghold in the mountains; there, in the tower of Hornburg, the dwelling of Erkenbrand, the master of Westfold, a number of the Rohirrim of that region had already sought refuge. The host enters Helm's Deep and prepares for defence; they are attacked by a large army of orcs and the wild men of Dunland, and despite all their valour the defenders must retreat into Hornburg and into the caves in the Deep. On the dawn of the following day, however, the host of the Rohirrim breaks out of the fastness and the dismayed orcs flee before it. At the same time Gandalf appears, as well as Erkenbrand and his army; the orcs are surrounded and driven into a newly-grown forest, which is actually a host of Huorns (Ents that had become tree-ish), and none come alive out of it.
Gandalf, Aragorn, Gimli, Legolas, Théoden, Éomer, and a part of the host of the Rohirrim ride towards Isengard. Gimli tells Legolas about the beauty of the Caverns of Helm's Deep, and they decide that some day they will go together and see both the Forest of Fangorn and the Caverns. The company travels through the forest of the Ents, then pass a great mound where the Rohirrim who fell in the nearby battles were buried. In the night they see a great shadow flying towards Isengard. Finally they reach the Ring of Isengard, where Saruman had long dwelt and turned it into a great fortress; but now it was defeated and ruined by the Ents. At the gates they find Merry and Pippin, enjoying all the comforts of food, wine and pipe-weed, and they direct Gandalf and Théoden towards the north wall, where they might find Treebeard.
Meanwhile Aragorn, Legolas and Gimli stay with the two hobbits, who tell them about their adventures since the breaking of the Fellowship of the Ring: the journey with the Orcs; the meeting with Treebeard; the attack of the Ents upon Isengard. The Ents are not much afflicted by arrows or axes, and they have broken down the gates and walls of Isengard; they could not harm the tower of Orthanc, though, and Saruman was trapped therein. The Huorns have formed a 'forest' all around Isengard, where all Orcs fleeing from Isengard have perished. The Ents have built dams and dug trenches, and directed the water from the Isen river towards Isengard, where is has flooded the underground caves and suffocated Saruman's fires. Gandalf has come and asked for help (hence the newly-grown forest after the battle of Helm's Deep); and later Wormtongue came, pretending to be a messenger from Théoden. Treebeard, however, whom Gandalf had warned against Gríma, gave him a choice of entering Orthanc or waiting for the coming of Théoden, and Wormtongue waded through the flood and entered the tower.
Gandalf, Théoden, Éomer, Aragorn, Gimli and Legolas go to the stairs of Orthanc to talk with Saruman, while the others wait a little farther away. Saruman has a sweet, hypnotic voice that can easily bring many under his influence. He addresses Théoden first, proposing peace and alliance between Isengard and Rohan and promising great benefits that could come of that. His voice charms the Riders, and it seems that it convinced Théoden too, but then he refuses and clearly states that there will never be such peace. Then Saruman talks to Gandalf and tries to convince him into uniting and using their power and wisdom to govern others. Gandalf laughs at this and then gives Saruman one last chance to come down and help them in their cause, or remain locked up in Orthanc. Saruman rejects this offer, and Gandalf causes his staff to break and casts him from the order of the wizards. A strange crystal globe, apparently thrown by Wormtongue, falls from a window; Gandalf takes it, hinting that it might be an object of great importance. The company says farewell to Treebeard (who promises that the Ents would guard Orthanc and prevent Saruman from escaping) and prepares to leave.
The company intends to ride to Edoras and starts in the direction of Helm's Deep. Pippin is very curious about the glass ball which he had picked up, and in the night when everybody is asleep he takes it from under Gandalf's arm. He cannot resist looking into it, and then drawing his eyes away from it, and a little later he falls back with a cry. Gandalf asks him what he has seen and done: in the stone he has seen the Dark Tower, and was questioned by Sauron. Sauron believed that the stone was still in Orthanc, and that the hobbit was Saruman's prisoner, and he only ordered Pippin to tell Saruman to hand out the prisoner to him, without setting further questions. The stone thus turns out to be a palantír, one of the seven stones used by the kings of old to communicate between distant places, and it was with this stone that Saruman could talk with the Dark Lord. A shadow passes over the camp: it is one of the Ringwraiths who are now riding upon horrible winged creatures, and it seems to be headed towards Isengard. Gandalf bids the others to ride on immediately with greatest haste, and he takes Pippin with him and rides with Shadowfax towards Minas Tirith as fast as possible.