The resurgent riders of Rohan have repulsed Sauron’s forces, who now sends a huge army to lay seige to Minas Tirith. Gandalf rides to rouse Denethor, much like he did for Theoden, but the severely addled Denethor tries to kill himself and his surviving son Faramir even as Sauron’s Orcs are bringing the gate down. Aragorn recruits the army of the dead, an accursed army that has been waiting for years to be called upon by the heir of Isildur.
Together, the riders of Rohan and the Army of the Dead break the seige of Minas Tirith and defeat Sauron’s forces. Not without losses, though. Eowyn is deeply wounded, and Theoden is among the dead. Aragorn inherits special healing powers as the rightful heir of Isildur, and uses these to heal Eowyn and Faramir. He learns of Frodo’s progress from Faramir, and leads the entire army to the black gate to distract Sauron. Sauron arrays his entire forces there as well.
Meanwhile Frodo, who has been captured by Orcs, is rescued by Sam. Frodo is much weakened, and Sam literally carries Frodo right up Mount Doom. When they finally reach the Cracks of Doom, Frodo is unable to resist the pull of the ring anymore. He initially refuses to destroy it and puts it on. Gollum attacks him at that moment, and literally bites off Frodo’s finger. But in the struggle, he falls, along with the ring, into the fires of Mount Doom and are both destroyed.
Immediately, Sauron loses his power and is crushed forever. His forces are in disarray and are easily defeated. Aragorn is crowned king, and the four hobbits return to the Shire, only to find that mercenaries under the control of Saruman have taken over. The four hobbits, having fought much more powerful adversaries, enlist the help of all the other hobbits and easily overcome the thugs. Saruman himself is killed by his minion Grima Wormtongue.
Sam marries Rosie Cotton and settles down, whereas Frodo joins Gandalf and all the elves on their journey to Valinor, the undying lands.
This is rightfully considered the peak of fantasy fiction, and a near perfect exemplification of the hero’s journey. There are those who cite certain scenes from the book as irrelevant or poor, but I find the book flawless.