I can understand the appeal of wanting to get as much use out of their exotic location filming (the island of Lanzarote, one of the Canary Islands) as possible, but it does mean that what we get is occasionally confusing as we switch between an alien planet and Earth, both of which more or less look the same (volcanic rock and blue oceans in the distance). It's only really a problem on the first viewing, but it's still there.
To be honest, the Earth scenes are more compelling in the first episode than the Sarn ones -- it's not really clear what's going on on Sarn, other than some form of heresy, but the Earth scenes have the Doctor and Turlough wandering around Lanzarote looking at ancient artifacts, using alien currency to pay for things, and rescuing drowning girls. This last action is performed by Turlough, who then brings the drowning girl -- Perpugilliam "Peri" Brown -- into the TARDIS to recover. And yes, that bikini she's wearing is quite racy for Doctor Who, but we also get Peri's father Howard shirtless and Turlough in a shirt and short swimming trunks, so at this point you can't complain too much.
But perhaps the most surprising thing about these two episodes is the return of Kamelion -- remember him? Not seen since The King's Demons129, here he's causing problems with some sort of alien signal, trying to get to some sort of "point of contact". The net effect of this is that we get to see Turlough be devious again, as he seems to recognize the distress signal being broadcast and clearly wants nothing to do with it. He's not terribly successful at stopping the signal though, as Kamelion takes them first to Earth, where there's an alien beacon, and then to Sarn, which bears the same mark on a column as on the beacon. "The Misos Triangle," Turlough says. "...It means there are people from Trion here. My home planet. This must be an old Trion colony." But while the Doctor and Turlough are out exploring, Kamelion turns first into Peri's stepfather and then into the Master. (Cliffhanger!) Yes, it seems the Master has managed to regain some control over Kamelion.
Part two, sad to say, isn't very interesting (which isn't to say it's bad -- it's just not terribly engaging). The best bits involve revelations about Turlough's background (the fact that he has a Misos Triangle on his arm like Malkon does is good, but the best bit comes near the end: "Doctor, you don't understand. I think this equipment [lying around the Hall of Fire on Sarn] came from my father's ship"), but these are few and far between. Instead we get extended sequences of Kamelion-as-the-Master chasing Peri across Sarn's surface while the Doctor and Turlough get involved (peripherally) in the holy war (well, argument, then) between believers and disbelievers of Logar, the Sarn Lord of Fire, but the disagreement isn't presented in a particularly interesting way -- it's instead supposed to be obvious that Timanov and the other believers are wrong. Still, at least the end of part two involves the Doctor finally encountering the Master (well, Kamelion, but he doesn't know that), which means that something more interesting may happen -- particularly since the Master has convinced Timanov that he is the Outsider that was prophesied to appear as the messenger of Logar, and that all the disbelievers should be burned...
129 One of the more entertaining things about the Wikipedia articles for the stories between Kamelion's two appearances is that every entry except The Five Doctors feels compelled to note that "[n]o explanation is given for companion Kamelion's absence from this story", as if this was of great concern or something. But does he count as a companion? He didn't show up in the companion roll call last story -- but, of course, neither did Leela...