Lamia is a clear attempt to fill an episode's weekly slot. It has no real continuity to recent episodes (Morgana even sits it out) and follows a rather boring and predicable storyline.
Plot: Grubby peasants, friends of Gwen, ask Arthur for aid, because a mysterious illness of 'sweating sickness' is sweeping over the men of their village. Arthur sends Merlin, Gwen and the knights to investigate. Once at the village, Merlin is convinced it's the workings of magic causing the sickness. As they ride back to Camelot to get Gaius, they meet up with a group of bandits and discover a ragged dressed girl, Lamia. Lamia is not as she seems, she's actually a 'creature of magic' half-girl-half-serpent (or rather...octopus?). She proceeds to bewitch the knights into doing her bidding, turning on each other and treating Merlin like a no-count. Despite their misgivings, both Gwen and Merlin (neither affected by Lamia) follow the knights on a downward path into Lamia's lair, where she slowly picks off the knights with 'kisses of death', leaving Merlin and Gwen (who picks up a sword this ep) to fight Lamia. Needless to say, Arthur, Agravaine and Gauis arrive in time, the sickness fades and the episode ends with Arthur jabbing Merlin and then back at the castle praising Gwen's bravery and getting a smooch.
The summery actually makes the episode sound more interesting than it is. The episode is 'filler', and has some rather boring bits of riding through the forest, discussing Lamia and then the rest is devoted to the knights trashing Merlin. The episode is flat and lifeless. There's no development, or anything that serves to be engrossing or entertaining. Lamia as a character herself is annoying, although I believe she was intended to be creepy? She lacks any motivation for her actions and just seems intent on killing men. The knights acting like their usual chain mail clad simpletons are easy prey for the ragged evil in the guise of dirty matted haired girl. The knights are used to such a poor advantage, you wonder why it merited all four of them. Could have easily gotten by with two. It's becoming tedious to watch Merlin belittled by the knights every chance given.
Another rather needless aspect is Agravaine's role in this episode. After bumbling around the throne room trying to get Arthur sidetracked from going after Merlin and Gwen, he saddles up and goes along for the ride. During the search, Agravaine preforms random acts of evil, like smoothing out the hoofprints to cover the trail and stalling. It's never apperent why Agravaine is doing this. Surely, since Lamia wasn't part of one of Morgana's schemes, Agravaine was at just as much risk as anyone else.
Lamia as a episode also brings to light one of the major shortcoming slowing eating away the entire premise of the series. Lamia is a creature of magic, devoid of any feeling or compassion, purely evil with living intent to be evil. Yet, we are supposedly waiting for a time with all creatures of magic are free.... Hmmm. Does that really work? It seems like liberating magic users and creatures could spell certain doom for Albion. Especially, since the creatures and users are always shown to be the greatest foes and dangers weekly. It stands to reason, un-banning magic might not be a good idea. Where that leaves the series premise, no clue.