Doctor Who: The Newbies Guide
This is the first story in Season 19.
The Fifth Doctor has just regenerated at the start of the story. This is our first introduction to him.
Adric is from the planet Alzarius in E-Space. He is a mathematical genius from a race with a high metabolism (which means he heals fast). In Full Circle he stowed away on the TARDIS.
Nyssa of Traken is the daughter of Tremas of Traken. In The Keeper of Traken she helped the Doctor and Adric to defeat the Melkur, a living statue that threatened their planet. The Melkur was actually the Master's TARDIS, and at the end of that story the Master took over her father's body. In Logopolis, the Master destroyed a large portion of the universe, including Traken.
Tegan Jovanka is an Australian air stewardess who accidentally stumbled into the TARDIS in Logopolis. She has a tendency to moan.
Recurring and Historical Characters
The Master is a Time Lord and an enemy and rival of the Doctor. In Terror of the Autons he arrived on Earth and allied with the Nestene Consciousness to attempt to conquer Earth. He was stopped by the Doctor and UNIT. He has since come into conflict with the Doctor on many occasions. In The Keeper of Traken he was reduced to a deathly husk, and then possessed the body of Tremas of Traken. In Logopolis he unleashed a wave of entropy that destroyed a large portion of the universe. His actions were also responsible for causing the Doctor to regenerate.
This story follows immediately on from Logopolis, in which the Doctor is forced to regenerate as the result of the Master's plans.