Arthur and Gwen are out for a romantic anniversary ride, and they’ve taken Merlin with them (for reasons that are unclear). However, their beatific calm is interrupted by an exploding sock, a ripped saddle, and an attack from the woods.
Arthur dispatches the two men who are trying to kill him (with a little help from Merlin’s magic), and the threesome head for Gaius’s house to dress Arthur’s wounds. It turns out Arthur’s saddle has been interfered with, and the blame falls on Tyr Seward, Arthur’s stablehand.
Tyr claims he’s done nothing wrong, but the evidence is stacked against him, and Arthur, unable to secure a confession, sentences him to death.
Naturally Merlin does not believe he’s guilty, and while he can’t convince Arthur, he does go off to investigate who else could have tampered with that saddle. Gwen, meanwhile, is happy to console Arthur that it definitely was Tyr, but let’s not forget the brainwashing Gwen had to undergo last week at the hands of Morgana, and the fact that she’s now officially Evil Gwen.
Merlin visits Tyr in prison. After exchanging a few pleasantries, Tyr confesses he saw someone fiddling with the king’s saddle a few nights before, and that they threatened his mother’s life if he told anyone what he’d seen. He’s reluctant to name the person he saw, for fear of further reprisals.
Merlin takes this information to Arthur, who resolves to speak to Tyr. But he’s distracted by Evil Gwen, who puts him off until morning. After all, what harm can it do?
Well, what if Evil Gwen was to sneak down to Tyr’s cell in the middle of the night, drug the guards, and stab him to death? That’d be pretty harmful, wouldn’t it?
The next day, Arthur bitterly regrets that he didn’t speak to Tyr when he had the chance. Merlin points out that the murderer must know Camelot to have snuck past the guards like that, and they all have a good puzzle over what’s happened.
Meanwhile, Evil Gwen has gone into the woods to meet Morgana and explain that her attempt to kill Arthur has failed. Morgana says she’ll come to see Gwen in Camelot with a new plan, and the two run off, chased by a patrolling Gwaine. Morgana hoiks him off his horse with magic, and they get away.
Evil Gwen overhears Merlin explaining to Gwaine his theories about there being a rotten insider in Camelot. Morgana, meanwhile is off sourcing a magical poison, called henbane and a sleeping draught from a potions master. Arthur confides in Evil Gwen that he’s becoming convinced that Merlin is right, and Gwen pledges to help him find the traitor in their midst (without the aid of a mirror).
Meanwhile, Gaius and Merlin are searching the forest, and have uncovered some evidence – a swatch of imported silk from an expensive dress. While checking Gwen’s dresses for holes, Merlin is interrupted:
Evil Gwen meets an old lady in the market, it’s MORGANA! And she hands over the poison, telling Gwen to find someone to blame so the suspicion does not fall on her. She chooses Merlin, then spikes Arthur’s drink. Merlin, meanwhile, has found the dress with the missing swatch, and its Gwen’s. He rushes to Arthur to tell him, but it’s too late, Arthur has drunk the sleeping draught and collapses, then Gwen puts the henbane in his ear.
When investigating the poisoning, Evil Gwen hints heavily that this can only have been done by someone with full access to the king’s every movement. And there’s only one person it can be: Merlin. Off he goes to prison, leaving Gaius to tend to Arthur, who is nearly dead. He goes to visit Merlin in prison, asking him to save the king with magic, and Merlin asks to be taken to Arthur’s bedside.
One old-man-ificating potion later, Merlin talks his way out of the cells by confusing the guards, and heads off to find Arthur, after a bit of a row with the cook. Merlin leads the guards a merry dance across Camelot, raising the alarm in his wake, then climbs up to Arthur’s window under cover of magical darkness.
Gaius has convinced Evil Gwen to hide, under guard from Gwaine, so Merlin has a clear shot at his king, with those magic healing hands of his. It’s touch and go for a while there, but soon Arthur rolls over and falls into a natural sleep. Merlin makes his way back to the cells, and, the following morning, he is freed.
Evil Gwen arranges for the potions master to confirm that it was indeed Morgana who bought the sleeping draught and the poisonous henbane, and therefore she is free of suspicion, for now, but Merlin knows she can no longer be trusted, and so does Gaius.