Review of all 6 episodes:
The Master teams up with reptilian creatures who have been dormant under the ocean for millions of years to try to conquer humanity (again). These creatures, which are dubbed Sea Devils by terrified humans, are relatives of the 'Silurians' or as The Doctor re-names them Eocenes. They were on Earth before humans and want to claim the planet back after eons of suspended animation in the ocean depths. The Doctor tries to broker peace whilst The Master whips up a war. Human leaders also have the appetite for war so trapping The Doctor in the middle.
This adventure is not flawless but provides superb entertainment and iconic scenes throughout. All episodes of this adventure are terrific fun, exciting, interesting, well written and acted with only very minor faults. The interest and quality levels never drop below 9.5/10 for me and this epitomises Pertwee era Who.
Pertwee himself is in majestic form demonstrating brilliant humour, dynamic action, warmth and intelligence. Katy Manning as Jo is as endearing as ever and Roger Delgado is at the top of his game as The Master. There is a tremendously fun sword fight between him and The Doctor as well as all the wonderful verbal jousting.
There are a few guards and sailors whose acting and stunts are underwhelming but they are a tiny minority, most of the support cast is convincing and the whole naval scenario is brought to believable life with superb location work and ships, guns, submarines, hovercrafts etc. It is extraordinary in ambition compared to many earlier and later eras of the show.
Three main guest characters add particular quality to the serial too: Edwin Richfield as Captain Hart is a great substitute for the Brigadier providing cynical but admirable support, Clive Morton as Trenchard is a fantastically rich character beautifully acted and Martin Boddey as Walker provides an incredible mixture of comedy and horrible, pompous warmongering in a terrific role in the last 2 parts.
The Sea Devils themselves, whilst looking unimpressive to modern audiences perhaps, were very well done for the day in terms of costume and voices. The performance of Peter Forbes-Robertson as their Chief is superb as is the concept of them as a race with links to the Silurians introduced a couple of seasons earlier.
Overall this is a really great adventure.
My ratings: Episodes 1, 2, 4 & 5 - 10/10, Episodes 3 & 4 - 9.5/10