Doctor Who: The Newbies Guide
This is the first story of Season 12 and the first story to feature the fourth Doctor.
The Fourth Doctor is a new incarnation of the Doctor. This is his first appearance.
Sarah Jane Smith is a journalist who, in The Time Warrior, sneaked into UNIT in search of a story and ended up accompanying the Doctor on a trip to the middle ages. Sarah has subsequently become the Doctor's travelling companion and an ally of UNIT.
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. By this point he has worked with the third Doctor for many years.
Sergeant Benton is a UNIT officer. He worked with the Second Doctor to prevent a Cyberman invasion in The Invasion. He has been working with the Third Doctor for several years.
Harry Sullivan is a new character. This is his first story.
Recurring and Historical Characters
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.