In this final installment of the Lord of the Rings trilogy, the former hobbit Smeagol turned psychotic Gollum (voice of ANDY SERKIS) is still leading hobbits Frodo Baggins (ELIJAH WOOD) and Sam (SEAN ASTIN) toward Mount Doom. It is there where Frodo, the bearer of the powerful and evil ring, hopes to destroy it. Unbeknownst to him, Gollum, a former ring bearer, has plans of his own.
Meanwhile, after fellow hobbit Pippin (BILLY BOYD) activates a device that convinces the Dark Lord that he's the ring bearer, wizard Gandalf (IAN McKELLEN) realizes he must take him to safety. Accordingly, they travel to Gondor where they meet Lord Denethor (JOHN NOBLE), the father of Faramir (DAVID WENHAM) and his late brother Boromir, and unsuccessful caretaker of the throne.
Things don't look good for him or his people as hordes of Orcs, under the control of the Dark Lord, are approaching and sure to defeat them. Accordingly, King Theoden (BERNARD HILL) has agreed to send help to Gondor. With Aragorn (VIGGO MORTENSEN), elf Legolas (ORLANDO BLOOM) and dwarf Gimli (JOHN RHYS-DAVIES) off to find ghostly reinforcements, the King, his nephew Eomer (KARL URBAN) and their army set off on their long journey.
Theoden's niece, Eowyn (MIRANDA OTTO), however, isn't pleased that she's been ordered to stay behind. Accordingly, she dons a disguise, grabs Pippin's best friend Merry (DOMINIC MONAGHAN) and joins the forces. Meanwhile, Aragorn's elf lover, Arwen (LIV TYLER), decides to live as a mortal rather than be immortal with her father, Elrond (HUGO WEAVING), a point that doesn't sit well with him.
With the various forces set to battle the Orcs and a powerful being known as the Witch-king, Frodo and Sam try to avoid Gollum's devious plans so that they can destroy the ring once and for all.