This is a classic Pertwee episode from the middle of Jon Pertwee's tenure as the Doctor. It has an interesting villain, two actually, counting the Master; good action sequences, and an interesting story.
As other reviewers have noted, this story is a sequel to Jon Pertwee's first season story, "The Silurians". The Sea Devils, are reptilian cousins of the Silurians, who have been awakened by undersea blasting to retrofit a 19th century seafort. The Sea Devils are part of race of intelligent reptiles that used to rule the Earth long before the evolution of man, and they want their planet back. The Master wants to help the Sea Devils destroy mankind, since the Doctor is so fond of humanity.
This is a good story for companion Jo Grant, who I remember being a somewhat annoyingly helpless character. She is able to evade prison guards, come up with a plan to free the Doctor, use her escapology coursework to free the Doctor from his handcuffs, and she even knows how to drive a hovercraft-something the naval captain seems to be incapable of. It is also a good story for the Master, who is in turns menacing and charming as it suits his needs. Jon Pertwee has grown quite comfortable in his portrayal of the Doctor, and this really shows his Doctor at his action-packed best. The supporting characters are well cast, and the story moves along fairly briskly for a 6-parter.
The British Navy's involvement with this story must be mentioned. The support given through locations, seacraft, sailors, weapons, and stock footage helped give this story a rich, almost movie-like look. Without the navy's involvement it would have been a much poorer-looking show, and the fact that it was provided gratis by the navy allowed the production team to use the money elsewhere.
The extras for this DVD aren't extensive, but they are good. There is a half hour making of feature, which is quite interesting featuring production team, cast and former naval personnel. The commentary with Director Michael Briant, producer Barry Letts and script editor Terrance Dicks, moderated by Andrew Cartmel is quite fun and interesting, and the information notes option is always fun and quite illuminating.
Going back to the commentary, I'm beginning to agree with another reviewer on a different Doctor Who DVD who said that Terrence Dicks should be brought in to do commentary whether he had anything to do with the story or not. Terrence Dicks is delightful on the commentary tracks. He is deservedly, quite proud of a lot that they accomplished during his tenure with Doctor Who, but that doesn't stop him from saying where they failed, when something's rubbish, or just plain funny, and he doesn't hold Doctor Who up on some godlike pedestal. Terrence Dicks and Barry Letts also have a long-standing friendship and working relationship based on respect and trust, and they feed nicely off each other on the commentary tracks, as they must have in their days as script editor and producer of Doctor Who. Michael Briant jumps right in with these two, and they have quite an interesting discussing going about the story, what worked, what didn't, and why certain things happened the way they did. Andrew Cartmel was rather wasted as a moderator, since these three all knew each other and had no trouble discussing things, and I'd hazard that he was actually more of a hindrance than a help, not really bringing up any interesting questions for the others to discuss until the last two episodes.
If you like Doctor Who, get this DVD, you'll enjoy the visit back to a time when the Doctor was exiled on Earth, and the scary monsters had to come to him.