Doctor Who: The Newbies Guide
Revenge of the Cybermen
This is the fifth and final story in Season 12.
The Fourth Doctor dresses in a rather bohemian outfit, notable for its long scarf. Unlike his previous incarnation, he is not willing to base himself on Earth with UNIT.
Sarah Jane Smith is a journalist who, in The Time Warrior, sneaked into UNIT in search of a story and ended up accompanying the third Doctor on a trip to the middle ages. Sarah has subsequently become the Doctor's travelling companion.
Harry Sullivan is a naval Doctor working for UNIT. In Robot he was responsible for examining the newly regenerated Fourth Doctor. At the end of that story the Doctor offered Harry a trip in the TARDIS. Harry accepted, not knowing that the TARDIS is actually capable of time and space travel. Harry is something of an upper class gentleman.
Recurring and Historical Characters
The Cybermen originally came from the planet Mondas, a twin planet to Earth. The planet somehow left its orbit, travelling into deep space. In order to survive its people started replacing their body parts with cybernetics. Eventually they became more machine than man, removing their emotions. Now much stronger than humans they seek only to survive and convert human beings into more Cybermen. The Doctor has encountered them numerous times.
In The Ark in Space the TARDIS landed on a space station occupied by humans who had gone into hibernation to escape a global catastrophe. In The Sontaran Experiment the Doctor, Sarah, and Harry went to check that Earth was suitable for re-colonisation. In Genesis of the Daleks the Time Lords intercepted their transmat beam back to the space station and left them with a Time Ring to get back to the TARDIS, which they had left on the space station.