The thing that immediately strikes you about Planet of Evil is how impressive that jungle set is. It's a riot of plants and colors, and it's all lit very moodily -- particularly on film. But what's perhaps even more impressive is that the character of the jungle is retained when they go to studio scenes shot on video; it still looks like the same set (albeit with more standard lighting). And so they've taken this really good jungle and decided to make it scary. So add one lost expedition and the Doctor and Sarah stumbling on the remnants, just as the reinforcements arrive, and you've got a nice old-fashioned tale of suspense. There's clearly something out there, killing people by sucking all the liquid from them (the disappearing effect is rather meh, but the dehydrated appearance of the bodies is first-rate) -- and unfortunately those reinforcements think the Doctor and Sarah are somehow responsible.
The Morestran spaceship does provide a nice contrast to the jungle set, even if it occasionally looks too much like a quick set that someone's put up in an hour, but the crew themselves disappoint by being rather one-dimensional; they don't trust the Doctor, and that seems to be their only motivation -- even to the exclusion of leaving Zeta Minor as quickly as possible. I guess once they think they've caught the culprits there's no need to worry -- and the Doctor and Sarah making an escape, only to have one of the Morestran guards attacked by a nifty Forbidden Planet-esque monster, doesn't exactly help their case any.
Part two gives us more of the same but adds into the mix some motivation for what's going on, thanks to the Doctor. The Morestrans still believe the Doctor's responsible for the deaths (and there seems to be a math error here: Sorenson says there were eight in his expedition, and he appears to be the only survivor; then we see O'Hara killed, which should make eight deaths, but Salamar keeps referring to seven deaths), but the Doctor's worked out that Zeta Minor is a gateway between universes, this one and one he refers to as one of antimatter, and that the antimatter universe isn't going to let the Morestrans take any of their universe away (in the form of mineral samples that Sorenson has collected). But the Morestrans desperately need energy, so Sorenson doesn't want to relinquish the minerals. However, their ship is unable to leave the surface, and then the antimatter monster attacks the ship -- which seems to convince the Morestrans that maybe the Doctor isn't responsible. The Doctor offers to negotiate with the other universe, but when he's at the black pool which seems to be the immediate gateway, the antimatter creature rises up and causes him to overbalance and fall into the pool (with another David Maloney freeze-frame cliffhanger)...