The Council decides that the One Ring has to be destroyed, which will also truly kill Sauron forever. But the One Ring can only be destroyed if it is thrown into the volcano Mount Doom in Sauron's land Mordor, where the One Ring was made.
The Council sends Frodo, the Ring-bearer, to destroy the Ring, and eight companions to help him. These nine people are the "Fellowship of the Ring": the four hobbits Frodo, Sam, Merry, and Pippin, the elf Legolas, the dwarf Gimli, the two men Aragorn and Boromir, and Gandalf the Grey.
At the beginning of the year 3019 the Fellowship begins their long journey. Before they go, Bilbo gives his sword "Sting" and his Mithril-armour to Frodo. With Gandalf as their leader, the Fellowship first goes southwards through the land "Hollin". They try to go east over the Misty Mountains through the "Redhorn Pass", but there is too much snow. The Fellowship decides to go under the mountains, through the old Dwarven mines, which are called Khazad-dûm or Moria. They almost manage to go through Moria without anything happening, but near the end they are attacked by Orcs. There is also a Balrog, a demonic evil creature from the First Age. Gandalf protects the Fellowship, but he and the Balrog fall into an abyss. Aragorn leads the rest of the Fellowship out of Moria.
Now east of the Misty Mountains, the remaining Fellowship comes to the forest Lothlórien, the land of the Galadhrim, a wood-elven people. The Fellowship is welcomed by Lady Galadriel and Lord Celeborn, the rulers of Lothlórien. The Fellowship stay for a while.
When the Fellowship leaves Lóthlórien, the elves give them boats with which they can travel down the river Anduin. Each member of the Fellowship also gets a present from Lady Galadriel.
The Fellowship travel down the river Anduin until they reach the Emyn Muil and the waterfall Rauros. There they stop to decide where to go now: south to Boromir's home city Minas Tirith, or east to Mordor. Frodo goes for a walk to help him decide. He meets Boromir, who says that the Fellowship should go to Minas Tirith. Boromir also begins to talk about using the Ring against Sauron. Frodo realizes that Boromir is influenced by the One Ring. Boromir tries to take the Ring from Frodo, but Frodo puts the Ring on, becomes invisible and escapes. Boromir becomes himself again and is very sorry for what he tried to do.
Frodo decides that he will go alone to Mordor, so that the Ring cannot influence or hurt anyone else. The rest of the Fellowship are worrying where Frodo is. When Boromir comes and tells them that Frodo has run away, all of the Fellowship go and search for him.
Sam thinks about the situation, and realizes that Frodo wants to go to Mordor alone. Sam runs back to the boats, and catches Frodo leaving. In the end Frodo and Sam go east to Mordor together, to destroy the One Ring, and hoping that their friends in the Fellowship will be well.
The book begins with Aragorn, who finds a dying Boromir. Boromir tells him that they were attacked by Orcs, who took Merry and Pippin with them. Boromir says he is sorry for everything and dies. Legolas and Gimli arrive. As a funeral, the three put Boromir's body in one of their boats, which they let fall down the waterfall Rauros. They find out that Frodo and Sam left them to go to Mordor, and that the Orcs that attacked them were Saruman's Orcs, who have taken Merry and Pippin. They decide to follow the Orcs westwards to save Merry and Pippin. West of the Emyn Muil they come into the land Rohan, home of the Rohirrim, the "Horse-lords". They meet a group of Rohirrim led by Éomer, nephew of King Théoden of Rohan. Éomer and his men have killed the Orc group on the border of the forest Fangorn, but did not see Merry or Pippin. Éomer gives them two horses, and Aragorn, Legolas, and Gimli ride to the Fangorn forest.
Merry and Pippin were caught and taken away by a group of Saruman's Orcs, the Uruk-hai. When the Orcs are surrounded and killed by the Rohirrim group, the two hobbits escape into the Fangorn Forest. There they meet Treebeard, an "Ent". Ents are giant tree-like creatures. Treebeard takes the hobbits with him, and they tell him what happens in the world outside Fangorn. The Ents have a meeting called the "Entmoot". During the "Entmoot", the Ents decide to fight Saruman. Treebeard, the other Ents, and the two hobbits go to Isengard, Saruman's home.
In Fangorn, Aragon, Legolas and Gimli meet Gandalf. He had died, but was sent back to Middle-earth as Gandalf the White, to further help the fight against Sauron. Gandalf tells them that Merry and Pippin are well. They then go to Edoras, the capital of Rohan. Gandalf tells King Théoden that they have to go to war against Saruman. Gríma Wormtongue, a spy and servant of Saruman, is cast out. The people of Edoras, led by Éomer's sister Éowyn, flee to Dunharrow in the White Mountains, while the army of the Rohirrim goes to their fortress at Helm's Deep. Aragorn, Legolas, and Gimli go with the Rohirrim warriors, but Gandalf leaves once more without notice. Saruman sends his army of ten thousand Uruk-hai, Orcs, and Dunland men to Helm's Deep. The next morning Gandalf arrives with another Rohirrim army. They defeat Saruman's army, and win the "Battle of the Hornburg".
Gandalf, his friends, and a small group of Rohirrim then go to Isengard, Saruman's home. But when they arrive it has already been destroyed by the Ents. They also meet Merry and Pippin there. They talk to Saruman, who still hides in his indestructible tower Orthanc. Gandalf cast him from the Order of Wizards. They find the palantír, which was thrown from the tower by Gríma. The Palantíri are magical black stones with which one can see far away things, and communicate with other palantíri. The group leaves Isengard and rests. Pippin steals the palantír and looks in, and because of this is discovered by Sauron. The group flees the place before the Nazgûl find them. Gandalf and Pippin go to Minas Tirith, while the Rohirrim and other members of the Fellowship go back to Helm's Deep.
Frodo and Sam are in the Emyn Muil mountains and journey eastwards to Mordor. While they are still in the Emyn Muil, they are attacked by Gollum. But the hobbits can defeat and catch him. Gollum has to promise to show them the way into Mordor. They go through the Dead Marshes and come to the Morannon, the Black Gate of Mordor. They cannot go in, but Gollum says he knows a secret way into Mordor.
Frodo, Sam, and Gollum travel south through Ithilien. There they see a battle between a group of Southrons from Harad and a group of Rangers of Gondor. The hobbits are caught by the Gondorian group, which is led by Faramir, who is Boromir's brother. But the next day Faramir lets the hobbits leave, because he also believes that the One Ring has to be destroyed.
Gollum leads the hobbits into Cirith Ungol in the Mountains of Shadow, but leaves the hobbits there alone. They are hunted by Shelob, a giant spider living in these tunnels. After almost escaping, Shelob stings Frodo, but is wounded and driven off by Sam. Sam believes Frodo is dead, and takes the Ring to continue the quest and destroy it. Frodo's body is found and taken away by a group of Orcs. Sam follows them. The leaders of the Orc group are talking, and Sam hears them say that Frodo is paralyzed, but still alive.
Gandalf and Pippin arrive in Minas Tirith, the capital of Gondor. There they meet Denethor II, Steward of Gondor, and father of Boromir and Faramir. Pippin becomes a member of the "Guards of the Citadel". Minas Tirith prepares for war.