When he learns of the existence of the last dragon egg, Merlin feels he has no option but to assist in its recovery.
After the high drama of the first three weeks, this is a return to the standard stand-alone episodes: Merlin finds himself in a situation where his magical knowledge put him at odds with Gaius and Arthur, does something stupid which Gaius and the audience can see coming a mile off, and then he has to put it right. This usually involves some form of lesson for the young warlock, and a move forward in the overall arc of the show.
Aithusa hits all of these points. There’s a very different guest starring spot from James Callis, (Battlestar Galactica’s Gaius) using his natural British accent, and a much deeper tone than we’re used to hearing from him. There’s some slapstick between Merlin and Arthur (although I wouldn’t go as far as the Radio Times in describing it as “worthy of Basil Fawlty”!) and a very nicely underplayed moment between Merlin and the Knights that emphasises how far all the relationships have developed over the show’s four years.
Given that last episode ended with Arthur’s coronation, it’s a little surprising that there’s no real recognition of Arthur’s changed circumstances, but it’s probably the right decision to have a lighter episode now before returning to weightier matters.
Verdict: Throwaway but enjoyable episode. 6/10
Paul Simpson
IF YOU HAVE COME HERE PLEASE REDIRECT YOUR BROWSER TO WWW.SCIFIBULLETIN.COM 
I, too, was surprised that nobody put up a fuss about the newly-crowned King taking off on a quest with his squire and knights.
Also had a geographical issue with the trek through the waterfalls at each end of the cave, which seemed awfully short to reveal a sprawling land behind it that, somehow, nobody had apparently stumbled upon in over 400 years!