Doctor Who: The Newbies Guide
The Android Invasion
This is the fourth story in Season 13.
The Fourth Doctor dresses in a rather bohemian outfit, notable for its long scarf. He is somewhat alien, willing to play the fool, and almost never gets on with authority figures.
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.
Recurring and Historical Characters
Harry Sullivan is a medical officer who was assigned to UNIT when the Doctor regenerated into his fourth incarnation. He spent some time travelling with the Doctor and Sarah after this point. Harry is very much an old-school gentleman.
RSM Benton is a UNIT officer. He worked with the Second Doctor to prevent a Cyberman invasion in The Invasion. He then worked with the Third Doctor for several years. He has worked with the fourth Doctor twice, in Robot and Terror of the Zygons.
The United Nations Intelligence Taskforce (UNIT) is an organisation devoted to protecting Earth from extra-terrestrial threats, In The Invasion, the second Doctor teamed up with them to prevent an invasion by the Cybermen. In Spearhead from Space, the third Doctor became their scientific advisor. The fourth Doctor fulfilled that role in Robot before taking off in the TARDIS, and again when called back to Earth in [tv]Terror of the Zygons[/tv[.