I just finished The Sea Devils. I thought it might be a dud after the first episode but it turned out pretty good.
Jo was really quite capable in this one, taking out guards, freeing the Doctor, spotting the Master and turning into a man and riding a motorcycle. Though I don't think we were supposed to notice her becoming a man. Maybe it's the power of pants. We got the third Doctor doing Venusian Aikido, delivering some dry humor and reversing the polarity of the neutron flow. We get the Master all crazy-eyed and compelling everyone. What could be better?
Lots of outdoor shots and varied locations help this one out. We have bases, subs, lairs, hovercrafts and more. We even get cars with no doors for some reason, must be a 70s/British thing...
Lots of trippy early synthesizer music as well. At one point, I think the Doctor even attacked the Sea Devils with a nasty feedback loop (or something...). There's something about early electronics that has a unique aura about it, I love all the glowing and clicking and humming and what not of all the electronics the Doctor and Master are using. Could he even hack a transmitter with modern electronics? I suppose with the sonic he can do anything now but it's cool seeing the Doctor hook the door up to electrical current. Of course, the Doctor using the sonic to get through a minefield was cool (and got to make more crazy noises).