The Doctor (Pertwee) comes across the Silurians again in this six part-story, The Sea Devils, that was written by Malcolm Hulke and aired from 26 February to 1 April, 1972.
Jo (Katy Manning) and The Doctor travel to a remote island off the coast of England where The Master (Roger Delgado) is currently incarcerated. What the duo don’t realize however, is that the renegade Time Lord is actually more close to running the place than being an actual prisoner. While there, they also decide to investigate the reports of ships disappearing in the area; with a secret Royal Navy base in the area as well, The Doctor can’t help but be drawn into the investigation.
Smack-dab in the middle of the area that the ships have been disappearing is an old sea fort that is being maintained as a sonar base, but it seems there is something else in the fort with them. What The Doctor discovers is another species of the Silurians, adapted for surviving in and under the water.
The Master, leaving his ‘cell’ behind, masquerades as a Naval commander so that he can make contact with the Silurians, and form an alliance which will be beneficial to them both, the extinction of humankind. The Doctor stumbles upon him on his return to a cell, and the two of them actually engage in a round of fencing, which, sadly, sees the Doctor being temporarily held prisoner, while The Master and The Silurians continue their plan. The Silurians do not wish to share the planet in a peaceful co-existence, they want to wrest the planet from humankind, little more than apes in their eyes, and either subjugate or destroy them.
As The Master and the Silurians make their move, The Doctor and Jo find themselves traversing a mine covered beach, pursued by an angry Silruian. The sonic screwdriver is put to good use, and actually detonates a number of the mines mostly harmlessly.
As Royal Navy warships converge on the area it seems like a full-fledged conflagration is going to occur, even though at this point, The Doctor, and The Master are working together on a device that may see the end of the humans, or the Silurians (depending on the polarity). The Doctor reveals that at one point he and The Master were in fact friends, and begins to build the old school mate theme that begins to run through the series.
Through all of this, The Doctor is still pushing for peace, but as the battle erupts, The Doctor is forced to destroy the Silurian colony in order to save humanity, and possibly the world, not only from the Silurians but from The Master as well.
Not to be kept, and finally free, The Master makes good his escape in a stolen RN hovercraft, a jaunty wave given to The Doctor, and a smile on his lips… he’ll be back!
Next time around, The Doctor encounters The Mutants!