Doctor Who: The Sea Devils

Arrested after the events at Devil's End, the Master has been incarcerated at a top security prison on a remote island off the south coast of England. When the Doctor (Jon Pertwee) and Jo Grant (Katy Manning) pay a visit to their old enemy, they find him a reformed character. But can the spate of mysterious shipping accidents and garbled reports of sea monsters in the local area really be a coincidence? The Doctor soon finds himself pitted against the Sea Devils, an ancient race of reptiles intent on eliminating humankind and reclaiming the Earth for themselves. Extras include commentary by director Michael Briant, producer Barry Letts, and script editor Terrance Dicks, and enough bonus featurettes to please even the most dedicated fans of Doctor Who.

The Doctor and Jo visit the Master in his high-security prison on an island off the south coast of England and hear from the governor, Colonel Trenchard, that ships have been mysteriously disappearing at sea.
Investigating, the Doctor learns from Captain Hart, commander of a nearby Naval base, that the sinkings have centred around an abandoned sea fort. He and Jo then visit the fort and are attacked by what one of the men there terms a Sea Devil - an amphibious breed of the prehistoric creatures encountered by the Doctor shortly after his exile to Earth.
The Master, aided by a misguided Trenchard, is stealing equipment from the Naval base in order to build a machine to revive the Sea Devils from hibernation.
The Doctor takes a diving bell down to the Sea Devils' underwater base to try to encourage peace. His efforts are frustrated by a depth charge attack ordered by a pompous politician, Walker, but in the confusion he manages to free a captured submarine and escape back to the surface.
The Sea Devils then capture the Naval base, and the Master has the Doctor taken back to their control centre, where he forces him to help finish the machine. The Doctor sabotages the machine and the two Time Lords escape together just as the base is destroyed in an explosion.

Directed by Michael E Briant
Costumes by Maggie Fletcher
Film Editing by Martyn Day
Incidental Music by Malcolm Clarke
Produced by Barry Letts
Script Editing by Terrance Dicks
Title Music by Ron Grainer and the BBC Radiophonic Workshop, arranged by Delia Derbyshire
Written by Malcolm Hulke