Book I: In the peaceful Shire, homeland of the Hobbits, Bilbo Baggins, main character of The Hobbit, decides to retire and depart for ever, leaving his entire estate to his nephew and heir Frodo Baggins. This includes a golden ring that causes possessiveness in anyone who owns it. The wizard Gandalf the Grey, family friend of the Bagginses, determines that this is nothing less than the One Ring, the Dark Lord Sauron's masterpiece. While it exists, Sauron cannot truly be destroyed; should Sauron ever regain it, ultimate victory will be his. Frodo, his cousins Merry and Pippin, and his gardener Sam set off for the Elven refuge Rivendell, hunted by Sauron's Ringwraiths the entire way. In Bree, they meet Strider, a mysterious ranger who guides them onwards. On the way, Frodo is wounded in an attack by the Ringwraiths, but they manage to reach Rivendell.
Book II: Frodo is healed by Elrond, Lord of Rivendell. A Council is held, consisting of the local elite and of various foreign messengers who came here independently, and much (historic and recent) is explainednote . It is made clear that the Ring cannot be used but has to be destroyed, in the only place it can be: the volcano Mount Doom in Sauron's land Mordor, where it was created. They go for the sneaky approach and send out a group of nine to fullfill the quest, the Fellowship of the Ring: the four hobbits; the wizard Gandalf the Grey; Legolas, an elven archer from Mirkwood; Gimli son of Glóin, a dwarf of Erebor; "Strider", who is really Aragorn, uncrowned king of Arnor and claimant to Gondor; and Boromir, heir to the Steward of Gondor.
Along the way, Gandalf is lost in combat against an ancient evil in the Mines of Moria because the Dwarves Dug Too Deep, leaving Aragorn in command. They rest in the Elven realm Lothlórien, and receive help and gifts before sent on their way down the river Anduin. When the decision comes to choose their further path, Boromir briefly succumbs to the influence of the Ring and attacks Frodo, who flees and decides to continue to Mt. Doom alone to spare his friends. Sam realizes this and is able to catch up with him, and the two hobbits leave together, headed east for Mordor.
Book III: Whilst Frodo and Sam depart, the other members of the Fellowship are attacked by orcs minions of the traitorous Saruman. Boromir dies defending Merry and Pippin, but both are kidnapped. The remaining three of the Fellowship decide to rescue them and follow the orcs west across the plains of Rohan. The orcs are slain on the border of Fangorn Forest by Riders of Rohan, while the two hobbits manage to escape into the woods. The three hunters, pursuing them thence, are met by Gandalf, who has been sent back from death as Gandalf the White to continue to guide the Free Peoples in their struggles. They go to the capital of Rohan and rally the king, Théoden, into openly fighting Saruman. While the civilian population led by the king's niece Éowyn seeks refuge in the mountain refuge Dunharrow, the Rohirrim army and our heroes go west to face Saruman's attack in the fortress of Helm's Deep; Gandalf excuses himself along the way with a promise to return. The human army survives the siege and, with the help of Gandalf returning with another army of Rohirrim and a strange ominous forest which seems to have sealed off the valley overnight, manage to vanquish Saruman's army of orcs and humans; afterwards, they travel to Saruman's fortress Isengard.
Meanwhile, the two hobbits Merry and Pippin, having escaped the orcs, are lost in Fangorn Forest, where they are found by an Ent, a walking sapient tree-like being. The Ents call a meeting and decide to fight Saruman; they go to Isengard and, with it emptied of his armies, lay waste to it. The Rohirrim and our heroes arrive after the battle, finding Saruman now confined to his impenetrable tower Orthanc and guarded by the Ents. Gandalf and Pippin leave for Minas Tirith, capital of Gondor. The others join the civillian Rohirrim at Dunharrow in the White Mountains. Aragorn, now joined by a group of his people, takes the Path of the Dead through the Mountains and acquires the help of the Army of the Dead, who have been cursed since having forsaken an oath to his ancestor.
Book IV: After having left the Fellowship, Frodo and Sam travel east. They are attacked by Gollum, who wants the One Ring, but are able to overwhelm him and force him to lead the way to Mordor. They traverse the mountains and marshland and arrive at the Black Gate, entrance to Mordor; as they cannot go in this way unobserved, Gollum promises to lead them on a secret path. They travel south through the Gondorian province Ithilien, in which they are briefly captured by Boromir's brother Faramir and his Men. They admit their burden, but Faramir, showing his true quality, understands their quest and releases them to continue on their way. Gollum leads them to a pass through the mountains bordering Mordor, but abandons them there, where they are attacked by the Giant Spider Shelob. Sam is able to drive her away, but Frodo has been stung and appears dead; Sam takes the Ring to finish the quest alone. He follows a group of orcs who find Frodo, and learns he is only paralyzed, not dead; Frodo is taken to a fortress while Sam is left outside.
Book V: Gandalf and Pippin arrive in Minas Tirith, capital of Gondor, and advise and witness the ongoing war against Sauron and preparation for more. Various armies from its provinces arrive, but they are small contingents. Most of them aren't above a few hundred, with the smallest being only a dozen, and are about only 3'000 all told, due to the armies of the south besieging their cities. With this few men at their disposal, even if they killed several of Sauron's soldiers for every one of their own, Gondor would still lose from sheer attrition. Gondor has sent for help from the neighboring kingdom of Rohan, but don't know if the message reached them, or if they can come. Sauron lays siege to the city.
Denethor, Steward of Gondor, sends Faramir out to defend the border at Osgiliath; when Faramir returns wounded, Denethor succumbs to despair, trying to have himself and Faramir burned on a funeral pyre. Faramir is saved by Gandalf and Pippin. As all seems lost, the Rohirrim arrive and the tide seems to turn. Then King Théoden is killed; Éowyn and Merry kill the Witch-King of Angmar, Sauron's lieutenant and leader of the Ringwraiths, but both are wounded. All seems grim once more as a fleet of ships from Sauron's allies arrive from the south, but they are manned by Gondorian soldiers. Aragorn used the Army of the Dead to win the battles raging in southern Gondor and capture the fleet waiting there, and used it to transport the now free Gondorian troops north. The focused might of the race of Men is able to defeat Sauron's armies. Afterwards, it is decided to take an army led by Aragorn northeast, to the Black Gate, to distract Sauron so that Frodo might have a better chance at his quest. At the Black Gate a great battle commences.
Book VI: Sam manages to free Frodo, taking advantage of infighting among the orcs, and returns the Ring to him. They travel across Mordor and finally reach Mount Doom, where they are once again attacked by Gollum. While Sam holds him off, Frodo goes inside the mountain... But there, he succumbs at last to the Ring, claiming it for himself and putting it on. Gollum fights him and bites off his finger with the Ring, finally reclaiming it, but, jubilant, missteps and topples into the volcano, destroying the One Ring and killing Sauron. In the resulting confusion the Battle at the Black Gate is won by the Free Peoples' armies. Frodo and Sam are saved from the eruption by the eagles, who had come to help in the battle.
The following chapters show how everything is tied up, along the way of tracking back the way home to the Shire. Aragorn is crowned King of Gondor and marries Arwen, daughter of Elrond; Éowyn, who had shown attraction (unrequited) to him, becomes involved with Faramir as both recuperate from their wounds. By the time the hobbits have returned to the shire, their companions have returned to their own lives and they find that Saruman has taken over their homeland in revenge, but the foursome are able to rally the Shire due to their own Character Development. Saruman is killed by his servant Gríma, who is himself shot by hobbit archers. Frodo, Sam, Merry and Pippin settle down to enjoy their Happily Ever After, with Sam marrying Rosie Cotton and Frodo working on completing the contents of a book bound in red leather, in which Bilbo set down an account of his adventures, which Frodo now augments with his own stories. Unfortunately, those who have been touched by the power of Sauron can have no rest on this side of the Sea, and in the end, Frodo and Bilbo, together with the bearers of the Elven Rings (Elrond, Galadriel, Gandalf) leave Middle-earth on an Elven ship headed for the Undying Lands of Aman, marking the end of the Third Age. Sam remains behind with his family and the red leather book, carrying on the knowledge and legacy of their struggles.
Uncounted years later, JRRTolkien found a copy of that book, the "Red Book of Westmarch," and translated it from Westron into British English. He published its contents as "The Hobbit" and "The Lord of the Rings". And Now You Know.