Doctor Who: The Newbies Guide
Terror of the Zygons
This is the first story in Season 13.
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 and an ally of UNIT.
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
Brigadier Lethbridge-Stewart is the commanding officer of the United Nations Intelligence Taskforce (UNIT). He worked with the Second Doctor to defeat threats to Earth on two occasions (in The Web of Fear and The Invasion. In Spearhead from Space, he teamed up with the Third Doctor to prevent an invasion by the Nestene and the Autons. During that story he persuaded the Doctor to work as his scientific adviser. He worked with the third Doctor for many years and witnessed his regeneration into the fourth Doctor.
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 and then, in Robot, worked with the fourth Doctor just after his regeneration.
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.
There's nothing you really ought to see, but the Doctor received a recall signal from the Brigadier at the end of Revenge of the Cybermen, alerting him to the fact that he was needed back on Earth.