Summary
The hobbit Déagol discovers the One Ring in a river while fishing with his cousin Sméagol. The Ring immediately ensnares Sméagol's mind, and he kills his cousin for it. Corrupted physically and mentally, he retreats into caves below the Misty Mountains and becomes known as Gollum.
Centuries later, during the War of the Ring, Gandalf leads Aragorn, Legolas, Gimli, and King Théoden of Rohan to Isengard, where they reunite with Merry and Pippin. Gandalf retrieves Saruman's palantír, and the group returns to Edoras to celebrate their victory at Helm's Deep. Pippin looks into the palantír, seeing Sauron and a burning tree. Gandalf deduces that the enemy plans to attack Gondor's capital Minas Tirith; he rides there with Pippin to warn Gondor's corrupt steward, Denethor. On Gandalf's instruction, Pippin triggers the lighting of the beacons to call for help from Rohan.
Frodo, who carries the Ring, and Sam continue their journey towards Mordor, unaware that Gollum, now their guide, plans to betray them and take the Ring for himself. The trio witness the Witch-king of Angmar, lord of the nine Nazgûl, setting off towards Gondor with his army of Orcs. Gollum frames Sam for eating food supplies and desiring the Ring; Frodo is deceived and orders Sam to go home.
As Théoden gathers his army, Elrond tells Aragorn that Arwen is dying, having refused to leave Middle-earth. Elrond gives Aragorn Andúril, reforged from the shards of King Elendil's sword Narsil, and urges him to claim Gondor's throne, to which he is heir. Aragorn travels the Paths of the Dead with Legolas and Gimli and pledges to release the ghosts there from their curse should they come to Gondor's aid.
Gollum tricks Frodo into the giant spider Shelob's lair alone. Frodo narrowly escapes and confronts Gollum, who falls down a chasm after a scuffle. Shelob discovers, paralyses, and binds Frodo but is wounded and driven away by a returning Sam. Sam mourns Frodo's apparent death and takes the Ring to finish the quest himself, but then realises his mistake when Orcs take Frodo captive. He rescues Frodo within Mordor, and the two continue towards Mount Doom.
Denethor sends his younger son, Faramir, on a suicide charge. Faramir returns gravely wounded; believing him dead, Denethor falls into madness. Gandalf marshals the defenders, but the enormous Orc army breaks into the city. Denethor attempts to burn himself and Faramir on a pyre, but Pippin alerts Gandalf, and they rescue Faramir; Denethor, set ablaze, jumps to his death.
Théoden arrives and leads his army against the Orcs. Despite initial success in the ensuing battle, they are repulsed by the Oliphaunt-riding Haradrim, and the Witch-king of Angmar mortally wounds Théoden; his niece Éowyn, disguised in male clothing, slays the Witch-king with Merry's help, before Théoden dies in her arms. Aragorn arrives with his Army of the Dead, who overcome Sauron's forces. Their oath fulfilled, the Dead are released from their curse.
Aragorn marches on Mordor to distract Sauron from Frodo and Sam's journey to Mount Doom, drawing all remaining forces to confront him. Gollum, having survived his fall, pursues Frodo and Sam, but Sam holds him back as Frodo enters the mountain. Frodo succumbs to the Ring's power and puts it on, but Gollum bites off his finger and reclaims the Ring, leading to a struggle before they tumble off the ledge. Sam saves Frodo while Gollum falls into the lava with the Ring, destroying it and Sauron forever. The lands of Mordor collapse, annihilating the Orc army. Frodo and Sam narrowly escape the erupting Mount Doom, before Gandalf and the eagles rescue them.
The surviving Fellowship members reunite in Minas Tirith. Aragorn is crowned King of Gondor and marries Arwen; everyone bows to the hobbits. The hobbits return to the Shire, where Sam marries Rosie Cotton. Four years later, Frodo, still suffering from trauma and a wound inflicted by the Witch-king, leaves Middle-earth for the Undying Lands with Bilbo, Gandalf, and the remaining Elves. He gives Sam the Red Book of Westmarch, detailing their adventures, bids farewell to Sam, Merry, and Pippin, and departs. A saddened Sam then returns home where he is comforted by his wife and two children.
Source: Wikipedia (CC BY-SA 3.0)