Bilbo Baggins throws a party for himself and his protÃ©gÃ©, Frodo. At the party, Bilbo announces that he is leaving his home to his heir, Frodo. He returns home and is met by his close friend, the wizard Gandalf. Gandalf insists that Bilbo remove the Ring that he has owned since the events of the previous adventure, chronicled in The Hobbit, and give the ring to his young heir. The Ring has special powers, the most obvious of which is to make the wearer invisible. Then Bilbo disappears.
For many years Frodo lives in Bilbo's home at Bag End. Just like Bilbo, Frodo appears not to he aged. In his fifties, he grows restless. One day Gandalf comes to Frodo and tells him that he is in danger. It seems that the Ring originally belonged to Sauron, the Dark Lord. Sauron wants the Ring back so that he can conquer the world. Sauron is using Gollum, an evil hobbit who also wants the Ring, to find out who has it and where it can be located. Gandalf tells Frodo that the ring is a corrupting power, and that anyone who uses it will ultimately be destroyed by it if they do not part with it. Further, he tells Frodo that the Ring can only be destroyed by tossing it into a volcano at Mount Orodruin. Frodo tries to give the ring to Gandalf, but the wizard tells him that he (Frodo) was chosen to bear the responsibility; it is his fate.
It is decided that Frodo will take the ring to the Crack of Doom in Mount Orodruin. He is to be accompanied by his friends Sam, Merry, and Pippin. As they travel, the Black Riders of Sauron pursue them. The Black Riders are bodiless horsemen who want the Ring. The travelers meet up with Aragorn, a friend of Gandalf, and together they continue their journey with the aid of some new companions.
Gandalf leads the companions through the mines of Moria. Gandalf battles a dreadful spirit and falls into an abyss. Aragorn becomes the leader. After many small battles, the company realizes their task will be very difficult. They meet Lady Galadriel, of the elves, and are given some assistance. Boromir, a representative from Gondor, tries to persuade Frodo to give him the Ring to take to his father and thus defeat their enemies. Frodo refuses and the two men fight. Frodo must use the Ring to escape Boromir. Boromir is instantly sorry he has been overcome by the allure of power. Frodo decides to travel alone, fearful of the consequences of his friends being corrupted. Only faithful Sam is allowed to accompany him.
Boromir is killed and given a hero's burial; everyone knows he never meant to fight Frodo. They attribute his sudden corruption to the Ring. Orcs attack and Merry and Pippin are taken captive. Aragorn, Gimli and Legolas pursue them, trying to save their companions. The riders of Rohan appear, having been summoned to help. They destroy the orcs near Fangorn forest but cannot find Merry and Pippin. Merry and Pippin have come into the area inhabited by Treebeard the Ent. He is the oldest living thing in the forest. He sustains them and rouses his troops to avenge the hobbits.
Meanwhile, Gandalf has come back from death as Gandalf the White. He approaches Aragorn, Gimli, and Legolas and tells them what has happened to him. He introduces them to King Theoden of Edoras. Together Gandalf and his men join Theoden in fighting the orcs and looking for Merry and Pippin. Along the way they defeat Saruman, who has been causing trouble and impeding Frodo in his quest.
Sam and Frodo are still making their way to the Crack of Doom. Gollum, who still craves the Ring, follows them. They capture Gollum and try to persuade him to forsake his evil and return to his old likable self as the character Smeagol. The three make their way through the forest until they are met by an army from Gondor, led by Faramir (Boromir's brother).
When Faramir releases them, Gollum leads Sam and Frodo into a trap--the lair of the giant spider Shelob. Frodo is wounded so badly that Sam fears he is dead. Gollum disappears. Sam decides to take the Ring and continue on in order to destroy it and fulfil Frodo's responsibility. Orcs come along and steal Frodo's lifeless body. By listening to the orcs, Sam discovers that Frodo is not dead but is merely drugged. He pursues them to save his good friend.
Gandalf and Pippin arrive in Gondor, where they meet Denethor (Boromir and Faramir's father). Pippin is befriended by Beregond. Aragorn has revealed himself to Sauron and decides to travel ahead through the Paths of the Dead in order to reach Sauron quickly and thereby assist Frodo and Sam. Eowyn, niece of Theoden, begs to be taken along but Aragorn refuses. He goes to the Paths of the Dead and offers the dead peace if they will fulfill their promise to fight against Sauron.
Merry is not allowed to accompany Theoden into battle, but the resourceful young hobbit hitches a ride with a fierce young soldier named Dernhelm.
Theoden, meanwhile, receives an urgent message to help Denethor at Minas Tirith. He refuses to let Merry join him, but Merry is offered a ride by a young rider named Dernhelm. Denethor is displeased with his younger son Faramir for having helped the Ring bearer rather than claiming the Ring for Gondor. He sends his son into battle, and when Faramir is seriously wounded, his repentant father goes mad with grief. He tries to set himself and his son's body on fire.
The story continues with the ride of the Rohirrim, allies of Gondor. They are attacked by Nazguls. Theoden falls beneath his wounded horse, but the young rider Dernhelm reveals himself as the fir Eowyn and kills the Lord of the Nazguls. Aragorn, meanwhile, rides into Gondor with the help of the forces of the dead and Sauron's evil minions are defeated.
Gandalf removes Faramir from the funeral pyre, but Denethor sets himself alight and dies. Merry and Pippin are reunited and Aragorn heals Merry, Eowyn and Faramir. The army then moves out to Mordor, where an emissary of Sauron produces Frodo's cloak and sword saying that if the troops do not withdraw, Frodo will be tortured. Gandalf snatches the objects from him and another war begins. Pippin saves Beregond from a troll, but faints just as the eagles come to their rescue.
Sam rescues Frodo and they make their way toward Mount Doom. Gollum, however, overtakes them and just as Frodo comes to the Cracks of Doom, Gollum obtains the Ring biting off his finger. Gollum is so excited about having the Ring back that he falls into the abyss, carrying the Ring with him. Mount Doom erupts with the force of the destroyed Ring. Sauron is defeated for good.
The eagles rescue Sam and Frodo from the erupting volcano. Faramir woos Eowyn Arwen marries Aragorn. The company then goes to Rohan for Theoden's funeral, and then Gandalf and the hobbits begin their homeward journey. The meet Saruman, who is unrepentant, then make their way to Rivendell, where they meet Bilbo. After this the hobbits reach the Shire, which has been completely transformed by Saruman and his servants. It is a barren military wasteland. The Shire hobbits manage to regain control and defeat Saruman. They restore the Shire with some help from Lady Galadriel.
The novel ends with Frodo's retirement. Elrond, Gandalf and Galadriel accompany him and Bilbo to the Grey Havens. Sam, Merry and Pippin return to live a full life in Hobbiton.