Synopsis

The three volumes of The Lord of the Rings are each divided into two 'books', making six books in total. Book I in The Fellowship of the Ring begins in the Shire with Bilbo's 111th birthday party, about 60 years after the end of The Hobbit, and his subsequent disappearance using his magic ring. Departing to journey once more, he left many of his belongings, including the ring, to his cousin and adoptive heir, Frodo Baggins. After 17 years of investigating, their old friend Gandalf the Grey confirmed that this ring was in fact the One Ring, the instrument of Sauron's power, for which the Dark Lord had been searching for most of the Third Age, and which corrupted others with desire for it and the power it held. Unknown to Gandalf, Gollum had made his way to Mordor, where he was captured and the little information he had about the Ring and its whereabouts (including the name Baggins) extracted through torture.
Sauron sent the nine Ringwraiths, in the guise of riders in black, to the Shire in search of the Ring. Frodo escaped, with the help of his loyal gardener Samwise "Sam" Gamgee and three close friends, Meriadoc "Merry" Brandybuck, Peregrin "Pippin" Took, and Fredegar "Fatty" Bolger. While Fatty acted as a decoy for the Ringwraiths, Frodo and the others set off to take the Ring to the Elven haven of Rivendell. They were aided by the enigmatic Tom Bombadil, and by a man called "Strider", who was later revealed to be Aragorn, the heir to the kingships of Gondor and Arnor, two great realms founded by the Númenórean exiles. Aragorn led the hobbits to Rivendell on Gandalf's request. However, Frodo was gravely wounded by the Ringwraiths at the hill of Weathertop. With the help of his companions and the Elf-lord Glorfindel, Frodo managed to enter Rivendell's borders by crossing the Ford of the river Bruinen. The Ringwraiths, in close pursuit, were swept away by an enchantment of the river when they entered its waters. Book I ends with Frodo losing consciousness.
Book II reveals that Frodo managed to recover under the care of the Half-elven lord Elrond, master of Rivendell. Frodo met Bilbo, now living there in retirement, and saw Elrond's daughter Arwen, Aragorn's betrothed. Later, much of the story's exposition is given during a high council, attended by representatives of the major races of Middle-earth (Elves, Dwarves, and Men) and presided over by Elrond. Gandalf told them of the emerging threat of Saruman, the leader of the Order of Wizards, who wanted the Ring for himself. In order to fulfil an ancient prophecy about the return of the King of Gondor and Arnor, Aragorn was going to war against Sauron armed with Narsil, the broken sword of Elendil. The shards of the sword had been kept safe in Rivendell through the intervening years, and were now reforged there and renamed Andúril. The Council decided that the only course of action that could save Middle-earth was to destroy the Ring by taking it to Mordor and casting it into Mount Doom, where it was forged. Frodo volunteered for the task, and a "Fellowship of the Ring" was formed to aid him, comprising his three Hobbit companions, Gandalf, Aragorn, Boromir of Gondor, Gimli the Dwarf, and Legolas the Elf, and with Frodo himself they were nine companions to go against the nine Ringwraiths.
The company journeyed across plains and over mountains, and ultimately to the Mines of Moria, where they were tracked by Gollum, who, having been released by Sauron, desperately sought to regain the ring he called his "Precious". When they were almost through the mines the party was attacked by Orcs. Gandalf battled a Balrog, an ancient demon creature, and fell into a deep chasm, apparently to his death. Escaping from Moria the Fellowship, now led by Aragorn, took refuge in the Elvish wood of Lothlórien, the realm of the Lady Galadriel and the Lord Celeborn. The Fellowship then travelled along the great River Anduin, and Frodo decided to continue the trek to Mordor on his own, largely due to the Ring's growing influence on Boromir and the threat it posed to the others. At the end of the book, Frodo attempted to continue his mission alone, but Sam was able to catch him at the last minute, and the two of them went off together towards Mordor.
The second volume, The Two Towers, deals with two parallel storylines, one in each of its books. Book III details the exploits of the remaining members of the Fellowship who aid the country of Rohan in its war against Saruman. At the beginning of the book, the remaining members of the Fellowship are attacked by Saruman's Orcs, and in the battle Boromir is killed and Merry and Pippin are kidnapped by the Orcs (Saruman, now turned traitor and seeking the One Ring himself, had sent them to capture the hobbits and bring them to him alive). Aragorn, Legolas and Gimli went off in pursuit of Merry and Pippin's captors. The three met Gandalf, who had returned as "Gandalf the White". He had defeated the Balrog at the cost of his life, but had been sent back to Middle-earth because his work was not finished. The four helped Rohan defeat Saruman's armies at the Battle of the Hornburg. Meanwhile Merry and Pippin, freed from captivity, helped the ancient, tree-like Ents to attack Saruman at his stronghold of Isengard. The two groups were reunited in the aftermath of battle. Saruman refused to repent of his folly and Gandalf cast him from the Order of Wizards, stripping Saruman of most of his power.
Book IV tells of Frodo and Sam's exploits on the way to Mount Doom. They managed to capture Gollum and convinced him to guide them to the Black Gate, which they found to be impenetrable. Gollum then suggested a secret path into Mordor, through the dreaded valley of Minas Morgul. While travelling there, the three were captured by Rangers of Gondor led by Boromir's brother Faramir, but managed to convince Faramir that the Ring was better off destroyed than used as a weapon. At the end of the volume, Gollum betrayed Frodo to the great spider Shelob, hoping to scavenge the Ring from Frodo's remains after she had consumed the hobbit. Shelob's bite paralysed Frodo, but Sam then fought her off. Frodo was taken by orcs to the nearby fortress of Cirith Ungol. Sam had thought his master dead, and saved the Ring, but was now left to find Frodo's whereabouts. Meanwhile, Sauron launched an all-out military assault upon Middle-earth, precipitating the War of the Ring, with the Witch-king (leader of the Ringwraiths) leading a large army into battle against Gondor.
The third volume, The Return of the King, begins with Gandalf arriving at Minas Tirith in Gondor with Pippin, to alert the city of the impending attack. Merry joined the army of Rohan, while the others led by Aragorn elected to journey through the 'Paths of the Dead' in the hope of enlisting the help of an undead army against the Corsairs of Umbar. Gandalf, Aragorn and the others of the Fellowship then assisted in the final battles against the armies of Sauron, including the siege of Minas Tirith. With the timely aid of Rohan's cavalry and Aragorn's assault up the river, a significant portion of Sauron's army was defeated and Minas Tirith saved. However Sauron still had thousands of troops available, and the main characters were forced into a climactic all-or-nothing battle before the Black Gate of Mordor, where the alliance of Gondor and Rohan fought desperately against Sauron's armies in order to distract him from the Ringbearer, and hoped to gain time for Frodo to destroy it.
In Book VI, Sam rescued Frodo from captivity. The pair then made their way through the rugged lands of Mordor and, after much struggle, finally reached Mount Doom itself (tailed closely by Gollum). However, the temptation of the Ring proved too great for Frodo, and he claimed it for himself in the end. While the Ringwraiths flew at top speed toward Mount Doom, Gollum struggled with Frodo for his "Precious" and managed to bite the Ring off Frodo's finger. Crazed with triumph, Gollum slipped into the Cracks of Doom, and the Ring was destroyed. With the end of the Ring, Sauron's armies lost heart, the Ringwraiths disintegrated, and Aragorn's army was victorious.
Thus, Sauron was banished from the world and his realm ended. Aragorn was crowned king of Gondor and married Arwen, the daughter of Elrond. All conflict was not over, however, for Saruman had managed to escape his captivity and enslave the Shire. Although he was soon overthrown by the Hobbits, and the four heroes helped to restore order and beautify the land again, it was not the same Shire that they had left. At the end, Frodo remained wounded in body and spirit and, accompanied by Bilbo, sailed west over the Sea to the Undying Lands, where he could find peace.
The Appendices contain much material concerning the timeline of the story, and information on the peoples and the languages of Middle-earth. Notably, Arwen, physically absent for much of the book, is dealt with in full here; her backstory and future with Aragorn are related.
According to Tolkien's timeline, the events depicted in the story occurred between Bilbo's announcement of his T.A. September 22, 3001 birthday party, and Sam's re-arrival to Bag End on T.A. October 6, 3021. Most of the events portrayed in the story occur in 3018 and 3019, with Frodo heading out from Bag End on T.A. September 23 3018, and the destruction of the Ring six months later on T.A. March 25 3019.