This episode reveals that though Merlin may be good with magic, he sucks with logic.
It would seem an evil sorceress type has dropped a mystical egg with a monster inside of it into Camelot’s water supply from a distant cave. It seems people who drink from that water die within 24 hours. Magic plagues! Those are the worst!
Mostly because it means Gaius can’t cure it. Uther is baffled, Arthur is searching for evidence of sorcery, and Merlin wants to cure people with magic. Gaius thinks that’s a bad idea, and it probably is, because Merlin is supposed to be undercover, and he barely keeps Arthur from seeing the magic book when the Prince comes looking for evidence of any magic anywhere.
But then Merlin does something dumb. He does it for good reasons, but it’s dumb. See, Gwen’s dad comes down with the mystery illness. So, he sneaks in that night and cures his friend’s blacksmith father. If he hadn’t left a magic poultice under the man’s pillow, it might have gone better.
But one guy gets well and that means people are suspicious. Someone must have used magic to cure the blacksmith? And who is the most likely suspect to be burned at the stake by morning? Gwen, of course.
Merlin tries confessing, but Arthur says he knows the truth that Merlin is smitten with Gwen.
He isn’t, but we can move on. As it is, Merlin and Gaius find a monster in the sewer system. That must be what’s making people sick, and a little research says the thing is an elemental of water and dirt. Maybe the other two elements of fire and air can stop it?
How do you do that? Well, you can ask the Great Cartoon Dragon, and he’ll say Arthur needs to be there to help. And Arthur does go along (so does Morgana), and yes, he is needed. See, someone had to hold the torch while Merlin stood behind him and conjured a wind, making a medieval flamethrower. And it kills the monster.
Was Arthur really necessary for that?
The fallout of all this? Well, Gwen is let out of jail since she is obviously innocent now. And Morgana also believes Merlin is smitten with Gwen. Damn. Sucks when your crush thinks you love someone else.
Oh, and the evil sorceress, Nimueh, knows Merlin knows magic and sees him as an obstacle.