You’ve probably noticed in my descriptions of Part 1 and Part 2 that the Doctor doesn’t do much in this story. He recognized Mondas before he even saw it and knew to expect the Cybermen, and once they’ve arrived, he doesn’t seem too concerned about them; he tells the bewildered General Cutler to wait it out. But it’s his companions Ben and Polly and the Antarctic base personnel who take the bulk of the exposition and action.
In this episode, more so than the previous two. William Hartnell was taken ill with bronchitis and went away for a week’s rest and recuperation. He isn’t even in this one.
Early on in Part 3, while the base is preparing its defense against the hundreds of Cyberships headed from Mondas toward Earth intent on siphoning off the planet’s energy and cyberizing the population, the Doctor (a stand-in with his back carefully toward the camera) collapses and is taken out of the control room. We’ll see him lying on a bunk under a blanket a little later in the episode.
While this was unfortunate for poor Mr. Hartnell, it provides some foreshadowing in character, suggesting that the Doctor’s health is deteriorating as well.
Everything that was scripted for the Doctor to have done in this episode, including most of his lines, is given to Dr. Barclay.