Doctor Who: The Newbies Guide
The Sea Devils
This is the third story in Season 9.
The Third Doctor has white hair, a large nose, and tends to dress as a society dandy. He has a love of technology, historical name-dropping, and vehicles. In The War Games the Doctor's people (the Time Lords) sentenced him to exile on Earth. In Spearhead from Space, he was appointed as scientific advisor to UNIT.
Josephine "Jo" Grant is the Doctor's assistant at UNIT. She failed General Science A Level, but has had training in escapology, cryptology, and explosives. In Terror of the Autons, on her first day on the job, she was hypnotised by the Master to assassinate the Doctor. She helped him against the Master on a number of other occasions.
Recurring and Historical Characters
The Master is a Time Lord and both an enemy and rival of the Doctor first seen in Terror of the Auton. He has clashed with the Doctor and UNIT a number of times. In The Daemons, he was captured by UNIT.
The Sea Devils are related to the "Silurians". Both 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 Doctor is strongly motivated by the events of Doctor Who and the Silurians.
Whilst the Master's capture was in The Daemons. all you need to know is that he has been captured and imprisoned.