When we last saw the Fellowship of the Ring, one of its members was dead, another - the wizard Gandalf (IAN McKELLEN) - seemingly fell to his death saving the others, and the rest had split up into various groups.
The bearer of the Ring, Frodo Baggins (ELIJAH WOOD) is still attempting to make his way to Mordor with his friend and fellow Hobbit, Sam (SEAN ASTIN), where they plan on destroying the ring once and for all. They're soon accompanied by Gollum (voice of ANDY SERKIS), an emaciated little creature who once possessed the ring. Suffering from a split personality disorder, he offers to help lead them to Mordor, but his evil side apparently has different plans. They eventually run into Faramir (DAVID WENHAM), the brother of a slain member of the Fellowship, while on their way.
Then there's Merry (DOMINIC MONAGHAN) and Pippin (BILLY BOYD), other hobbits who were captured by the forces commanded by evil wizard Saruman (CHRISTOPHER LEE) who still plots on ruling Middle Earth and is building an army of monstrous Uruks to assist him. The hobbits eventually escape, but soon find themselves under the wary eye of Treebeard (voice of JOHN RHYS-DAVIES), an ancient and gargantuan tree-shepherd who wonders if they work for Saruman and are responsible for the heavy deforestation that's occurring.
Meanwhile, human Aragorn (VIGGO MORTENSEN), elf Legolas (ORLANDO BLOOM) and dwarf Gimli (JOHN RHYS-DAVIES) are on their trail, hoping to save the hobbits. They eventually make their way into the Kingdom of Rohan where King Theoden (BERNARD HILL) has been put under a spell by Saruman and is controlled by his court advisor, Wormtongue (BRAD DOURIF).
With the king's son dead, his niece and nephew, Eowyn (MIRANDA OTTO) and Eomer (KARL URBAN), now doubt the king's abilities, particularly with Saruman's forces closing in. Once the King is freed from his spell, things improve and they head off for the cliff fortress at Helm's Deep where they hope to make a stand against the approaching hordes. Eowyn sees much in Aragorn, but he still longs for Arwen (LIV TYLER), his elf lover, who must make a decision about whether to leave with the rest of the departing elves, or stay with him and face his mortality.
With Saruman's massive forces making their way through the land and toward the fortress, the various groups continue on their quest to defeat the evil wizard and eventually return the Ring to its point of origin where it can be destroyed once and for all.