Doctor Who: The Newbies Guide
Warriors of the Deep
This is the first story in Season 21.
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 Enlightenment he cancelled the deal.
Recurring and Historical Characters
The Silurians are reptilian humanoids who evolved on Earth many millions of years ago. Their civilisation detected the object that would become Earth's moon approaching the planet and put themselves in hibernation in order to survive the predicted global catastrophe. However, they did not revive after the moon arrived.
In Doctor Who and the Silurians, the Doctor and UNIT discovered a group of Silurians who had been woken up by human activity. The Doctor attempted to make peace between human and Silurian. He was prevented from doing so partly by hardliners amongst the Silurians, and partly due to his friend the Brigadier choosing to blow up the Silurians' base. The Silurians we have previously met have a third eye which gives them some form of psychic powers.
The Sea Devils are aquatic cousins of the Silurians. In The Sea Devils, the Doctors enemy the Master made contact with a group of them and tried to provoke a war between them and humanity. The Doctor prevented this outcome.
You might want to see the Doctor's earlier encounters with the Silurians and the Sea Devils in Doctor Who and the Silurians and The Sea Devils
The Missing Adventures novel The Scales of Injustice was written as a prequel to this story. The novel explains away a number of discrepancies between this story and the two stories that it is a sequel to. It is, of course, entirely optional reading.