The Doctor (William Hartnell), along with his granddaughter Susan (Carole Ann Ford) and Ian and Barbara (William Russell and Jacqueline Hill), two teachers who had followed the oddly-behaving Susan home (at the end of the previous serial), find themselves on a strangely metallic planet and in the middle of a conflict between two groups of survivors of an ancient nuclear holocaust: one being decent, pacifistic humanoids, the other being malignant creatures who have adapted to the lingering radiation by encasing themselves in armoured exoskeletons. This was the second adventure of the Doctor and chronicles his first encounter with the nasal X-terminators who would serve as his nemeses across time and space for the next 60 years. Terry Nation's story is quite good and has a bit more of an edge than later, more light-weight 'classic' Whovian adventures. The nature of the Doctor was still being established and Hartnell's characterisation, while properly supercilious and arrogant at times, is more vulnerable and less omniscient than later incarnations. At seven episodes, the series is a bit stretched (notably the sequences in the caves) and the ending is a bit underwhelming (probably because the show's budget was insufficient to set up the epic final conflict that the build-up deserved). IMO, the addition of the Daleks to the Doctor's universe and the introduction of 'regeneration' (in 1966's 'The Tenth Planet') were the foundational events that led to the BBC series becoming a decades-spanning phenomenon. Essential viewing for all fans of the venerable Time-lord.