This Body is Wearing a Bit Thin
One of the next episodes has the Tardis land at the South Pole. Ben and Polly are hopeful they can catch a ride back to London, but soon discover they are in 1986, not 1966. Despite the cold weather outside, the Doctor encourages them to dress warmly so they can go out and explore. Shortly they run into a raised periscope from a base hidden beneath them.
Considering it's a South Pole base, it's not surprising the Tardis crew are immediately questioned. Things turn worse when a space capsule the base has been monitoring begins to lose power. The Doctor realizes what is happening but no one takes him seriously. A rogue sister planet of Earth, 'Mondas' is discovered and draining the energy of the space capsule and Earth itself.
The station is soon visited by creatures from Mondas. They call themselves Cybermen and were once human but have replaced all organic parts of themselves (except the brain) with plastic and steel. They have also stripped themselves of all emotions, such as love and empathy but claim to be logical. (It seems they've also lost all knowledge of fashion sense).
As much as he loved playing the Doctor, the strain finally caught up with William Hartnell. In fact, he became so ill before the filming of the third episode of his final story they had to carry on without him. Ben continued speaking for the Doctor and stressed how they didn't need to fight the Cybermen; if they were patient Mondas would draw too much energy from Earth and burn itself out.
The Doctors prediction comes true and Mondas burns itself out. The Cybermen also wither away, as if the enormous draw of energy was too much for them as well. Suddenly the Doctor realizes the stress has finally gotten to him and quickly retreats to the Tardis. With William Hartnell leaving, how do the producers decide to carry on? The answer comes as the Doctor collapses to the floor, suddenly his features begin to change. Ben and Polly are clueless as to what is happening, as are the viewers.
WHO: The Cybermen
PLANET OF ORIGIN: Mondas
GOALS: Self preservation through exploitation of other races.
WEAKNESSES: Their own weapons and home planet being destroyed. (More weakness to be discovered).
IN TWO WEEKS: The Power of the Regeneration
NEXT: The First Doctor in Review