Well this is best categorized as a reunion episode, hardly anything happens here. The opening bit with the Doctor and company escaping from the ventilation shafts with one of the Daleks in pursuit was good but the episode then goes slow from there. The characters reunite and some of the Thal rebels fight among themselves as in previous episodes.
We do see the mutated organic beasts helping the Daleks, and we see the Daleks planning to wipe out the remaining life forms with a deadly gas but that doesn't form into anything till the next episode.
An ok episode but since these are the "middle" episodes, a lot of action is left out.