Doctor Who: The Newbies Guide
This is the fifth story in Season 19.
The Fifth Doctor is physically younger than his previous incarnations. He wears Edwardian cricketing clothes with a stick of celery on the lapel. He is less eccentric than his previous incarnations.
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 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 Revenge of the Cybermen, he discovered that gold dust clogs their breathing apparatus, and is fatal to them.