A main character is desperately ill.
Gaius hasn't seen anything like it before, but can't believe it could have anything to do with sorcery like last week.
COINCIDENTALLY, some complete strangers have arrived at the castle and are welcomed with open arms by King Uther for some reason, and are allowed to stay in the room next to his only son and heir. Why not, eh? They have some kind of symbol/source of their power which they occasionally leave unattended.
Merlin immediately thinks they're the best thing since sliced bread (which won't be invented for 500 years, according to someone ranting on Points of View the following week).
Later, Gaius comes to the conclusion that the cause of the illness can only be SORCERY. Unsure, Merlin leaves in a hurry. "Don't go using any of that fucking sorcery, now, will you?!", Gaius reminds him, as usual.
Merlin uses his SORCERY to break into the mysterious visitors' chambers. He slowly approaches their mysterious source of power/christmas ornament, reaching it just as they DRAMATICALLY burst in. "What're you doing in here?", they ask, eyeing their source of power.
"Oh, just, um.. are these YOUR quarters? LOL, I thought Arthur had done some decorating.. oh, he's next door. Right... I know nothing, que?" Merlin leaves and returns to Gaius's workshop. It turns out he stole some incriminating evidence.
Cut to Merlin in the stocks. He hasn't done anything wrong, the producers are just taking an opportunity to hurl anachronistic veg at him.
Morgana hasn't been in the episode yet, so the next scene takes place in Uther's court. Morgana's there. She has breasts. Uther's off being grumpy somewhere else in the castle, so Merlin has to explain the situation to Arthur. Arthur doesn't believe his stupid retarded servant even though he saved him from a sorcery induced coma last week.
Morgana says something indecipherable. Arthur agrees, looking at her breasts. The evil houseguests do seem a little peculiar.
Uther comes in to the hall "what are you all doing here?" Cutting them off as they plead their case, he forbids them to investigate the mysterious strangers.
Morgana investigates the mysterious strangers, while Arthur goes on a lacklustre "quest" to get the cure for Gwen's illness. But he's not doing it so Morgana will like him more, no siree bob!
Merlin returns to Gaius's chambers where Gaius has made a brilliant discovery. The evil houseguests want to KILL King Uther. Gaius works a reference to the King's sorcery ban into the conversation, reminding Merlin that if Uther dies, he can probably fulfill his destiny.
The dragon is absolutely no help.
Merlin decides to help protect Uther, since it's worked out fairly well so far (except that time he tried to execute Gwen. And Morgana. And Merlin. And a little boy). Using Gaius's notes from wikipedia, the gang bring the villains to justice.
King Uther is still incredulous, telling them all to fuck off so he can sulk alone with Morgana.
Just as the villainous guests are about to be let free, they suddenly bring out their SOURCE OF POWER.
Merlin attempts a spell as Arthur draws his sword and attempts to fight off the bad guys, but they use sorcery against him. Just in the nick of time, Merlin manages to get the spell just right and a box of Ardal O'Hanlon Dr Who figures falls on them in a surprisingly graphic teatime shedding of blood and unwanted molten plastic.
Uther gives a short speech, reminding everyone how not using sorcery makes them stronger against people who do use sorcery somehow, and thanks everyone for being so loyal. He's an idiot.
Morgana's breasts thanks Arthur for saving Gwen as they walk down the corridor that's reserved for touching character moments. End credits.