It was bound to happen sooner or later. I mean, nobody's ever made a television series without a duff episode or two, and every seven-parter has its share of padding here and there. It just so happens that this is the episode where all of the padding went, and a good share of the awful acting as well. We've had a lovely run of great, tense sequences with lots of intelligent moral dilemmas and real intense emotions, and we'll get those things again. It's just one utterly missable episode, after all. Not so terrible in the scheme of things.
But it is utterly missable. It's a complete stall; the Doctor's group sits around for most of the episode playing ineffectual pranks on the Daleks before getting captured, while Ian's party wanders around in a series of caves running into literal as well as figurative dead ends. As for the Daleks, the only thing they manage to achieve in thirty minutes of screen time is figuring out that it wouldn't be very exciting if the heroes had only twenty-three days to stop them from wiping out all life on the planet. Oh, and they get a scene where they just straight up do Nazi salutes and announce that tomorrow belongs to them. Terry Nation's thirty-three year long analogy begins here, folks.
Perhaps the episode wouldn't have been so bad if Marcus Hammond had made a better job of his performance as Antodus. The whole tension of the episode, at least on paper, revolves around Antodus' slow, inexorable nervous breakdown that threatens to undermine the success of Ian's expedition. (It should be noted that even on paper, this isn't particularly enthralling, because Antodus is never entrusted with any particular responsibility with which he could fail and cause danger to the others. He more or less hangs out at the back whining, and the primary danger seems to be that they could at any moment lose a bit of the deadweight slowing them down. The only way Nation can contrive even a small amount of danger is to literally tie Ian to Antodus and make his dead weight a concrete threat instead of merely a metaphor.)
But even with its other flaws, Hammond's performance drags the whole thing down. His plea to Ganatus to turn back, which should be a total breakdown into quivering panic, sounds more like he's advising that they're all out of cucumber sandwiches and it may just be time to retire to the clubhouse for gin and tonics before the weather gets too bally rough to get in another round of croquet. He has no urgency, no fear, and no real character in the scene, and as a result you're utterly uninvested in whether he succeeds or not. And if Antodus, the one potentially interesting aspect of the episode, fails...well, you're not left with much to work with. It says a lot that even the cardboard Dalek standees couldn't lower the previous episode to this story's level.
(But wow, did they try.)