Unfortunately this is not the send off that William Hartnell deserved - he is barely present in the story. He spends most of the first two episodes sitting around and doing nothing other than uttering the odd line and he is not even present in the third episode - apparently Hartnell and producer, Innes Lloyd had clashed several times and the producer decided to replace the lead actor, effectively sacking Hartnell. The actor phoned in sick and refused to come to work, likely thinking this would sabotage the show, but the production carried on without him.
'I didn't leave the show willingly,' Hartnell wrote in answer to a letter from fan, Ian K McLachlan.
Of course conventional wisdom is that Hartnell was forced to finish due to ill health and yet he went back into theatre work immediately afterwards.
OK the plot - set in the then far off world of 1986 the Cybermen attack the International Space Command in Geneva. The dastardly Cybermen have a plan to drain the power from Earth in order to save their own planet, Mondas, but of course they fail and their own planet boils away into space. A problem with the story is the Doctor's regeneration is hardly explained and happens too quickly at the climax but then of course the limitations in the way the story was show is likely down to the fact that Hartnell wouldn't play ball and by all accounts the shoot was a chore for all concerned.