When we first meet Théoden, he is under the power of Saruman by proxy; specifically, through the influence of the traitor Gríma Wormtongue. With Gandalf's help, Théoden throws off Saruman's influence and leads his people first to the famous battle against Saruman's Uruk-hai at Helm's Deep, and then to the aid of Gondor in the war against Sauron. Théoden dies in the Battle of the Pelennor Fields.
Where now are the horse and the rider? Where is the horn that was blowing?
Where is the helm and the hauberk, and the bright hair flowing?
Where is the harp on the harpstring, and the red fire glowing?
Where is the spring and the harvest and the tall corn growing?
They have passed like rain on the mountain, like a wind in the meadow;
The days have gone down in the West behind the hills into shadow.
Who shall gather the smoke of the deadwood burning,
Or behold the flowing years from the Sea returning? -- Théoden, The Two Towers