This one's cute, but the ending seemed a little too easy.
The Merlin Wiki has a summary here.
First of all, Arthur's role in this is very sweet. I love that early scene between Arthur and Merlin in which Arthur admits that Merlin is in fact very brave and loyal. I also love how he reacts to the more "efficient" George. His thinly-veiled alarm at being subjected to such a sycophantic man-servant is hilarious. And by the way, it also reveals one of Arthur's redemptive qualities. Due to his noble breeding, he can be a bit of a jerk sometimes -- but really, when you get right down to it, Arthur is happiest when he's surrounded by people who occasionally tell him to sod off. Indeed, he thrives on those confrontations.
Secondly? FAIL, Morgana. FAIL. Could you have selected a worse assassin? I think not. But, of course, that means more entertainment for us. Merlin's clumsy attempts to kill Arthur are very funny in a dark way. They remind this reviewer of those Roadrunner cartoons in which the Coyote comes up with these elaborate plots to capture the Roadrunner and repeatedly fails because the Roadrunner is just so damned lucky -- and also has the uncanny ability to defy the laws of physics.
So, yes -- although this episode also sets up the inevitable discovery of Agravaine's treachery, for the most part, it stays on the lighter side. And there's nothing wrong with that, really. The best loaves are those that are leavened. The only flaw I can perceive in the script is the pat ending. The Fomorroh are billed as creatures of dark magic that are difficult to kill, so you would think Merlin would've had to do more than say a few magic words and throw the mother beast into the fire. But oh well. Regardless of the ending, this episode is still basically enjoyable.
As I noted above, this is a pretty fluffy script, but that doesn't mean it's bad.
The performances are also decent. I have no huge complaints here.