Doctor Who: The Newbies Guide
This is the fifth story in Season 20.
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. In Earthshock he was unable to save his companion Adric from dying.
Tegan Jovanka is an Australian air stewardess who accidentally stumbled into the TARDIS in Logopolis. She has a tendency to moan. In Time-Flight she was left behind in her native time, but in Arc of Infinity she happened to run into the Doctor and rejoined him on his travels.
Vislor Turlough is an alien who was exiled to Earth. He was posing as a schoolboy at Brendon School. In Mawdryn Undead he made a deal with the Black Guardian to kill the Doctor in exchange for returning home. He did not manage to do so during that story but did stow away on the TARDIS and then ask the Doctor to allow him to join him on his travels. In Terminus he failed to kill the Doctor and the Black Guardian gave him one more chance to complete his task.
Recurring and Historical Characters
The Black Guardian is a cosmic entity, being the equal and opposite to the White Guardian. In The Armageddon Factor the Doctor prevented him from getting his hands on the Key to Time. In Mawdryn Undead he made a deal with Turlough to transport him back to his home planet if he killed the Doctor.
Mawdryn Undead shows the original deal between Turlough and the Black Guardian. Terminus shows us Turlough's failure to kill the Doctor.