Two Hobbits, Sméagol and Déagol, are fishing when Déagol discovers the One Ring in the river. Sméagol is ensnared by the Ring, and kills his friend for it. He retreats into the Misty Mountains as the Ring twists his body and mind, until he becomes the creature Gollum.
Centuries later, during the War of the Ring, Gandalf leads Aragorn, Legolas, Gimli, and King Théoden to Isengard, where they reunite with Merry and Pippin. Gandalf retrieves the defeated Saruman's palantír. Pippin later looks into the seeing-stone, and is telepathically attacked by Sauron. Gandalf deduces that Sauron will attack Gondor's capital Minas Tirith. He rides there to warn Gondor's steward Denethor, taking Pippin with him.
Gollum leads Frodo Baggins and Samwise Gamgee to Minas Morgul, where they watch the Witch-king, leader of the nine Nazgûl, lead an army of Orcs towards Gondor. The hobbits begin climbing a stair carved in the cliff face that will take them into Mordor via a 'secret way' - unaware that Gollum plans to kill them and take the Ring. The Witch-king and his forces strike and overwhelm Osgiliath, forcing Faramir and his garrison to retreat to Minas Tirith.