Where the opening two-parter of this season ramped up the show’s scope in terms of effects and battle sequences, The Crystal Cave does the same emotionally, with Merlin’s ability to make things worse by his actions leading to a life-threatening injury for Morgana.
One of the criticisms of this version of Arthurian legend is the liberties it’s taken with some of the characters, and this episode goes some way to bringing the show back in line, as Uther is forced into a revelation that explains much of his behaviour to fans of the show who aren’t familiar with the myths. It affords Tony Head an opportunity to have a scene with Colin Morgan that doesn’t involve him sentencing Merlin to death/imprisonment (delete as applicable).
There’s a subtle shifting in the relationship between Merlin and a number of the other characters as well, including Arthur and the Dragon, as the young warlock finally begins to recognise the responsibilities that are incumbent upon him. Morgan has developed the ability to say much with just a glance which makes these scenes much stronger than they might have been earlier in the show’s run.
With the idiocies of the goblin episode hopefully a complete aberration, this is shaping up into the best season yet for Merlin. 8/10