On their wedding anniversary, Arthur and Gwen ride out of Camelot - accompanied by Merlin - for a picnic, only to be ambushed by bandits, with Arthur being thrown from his horse.
Evidence implicates Arthur's groom, Tyr Seward (John Bradley), but he pleads his innocence in the face of a death sentence for treason.
Tyr confides in Merlin that he knows who actually tampered with Arthur's saddle, but he is too afraid to speak their name. The next morning he is found dead in the cells.
The murderer was, of course, Gwen, who is now working for Morgana. Her next plan is to poison Arthur and put the blame on Merlin.
I have to say I'm not feeling particularly engaged with this "traitorous Gwen" turn-of-events. It certainly didn't help that A Lesson In Vengeance started with an ambush - just like The Dark Tower - but the whole sub-plot of a viper in the bosom of Camelot just has too many similarities to when Morgana was still in the castle, under the rule of Uther.
And while it was great to see Game Of Thrones' John Bradley here, there's no avoiding the obvious fact that Tyr Seward and Samwell Tarly are almost interchangeable as characters, which pretty much sums up the problem with the whole episode: a strong sense of déjà vu!
Even the presence of the herbalist Sindri (Tony Guilfoyle), with his nice neat woodland cottage/supermarket full of racks of potions and tinctures, felt just a bit pat. There was no sense that he had "always been there", but rather that this potions outlet had appeared simply for the sake of this story.
This season of Merlin started off so fantastically that it's a pity the scripts have fallen back into retreading old ground. While Arthur and Gwen have had three years of peaceful reign - in the gap between Season Four and Season Five - it would have been nice for us to have enjoyed a whole season of Gwen as queen before getting her "turned" by Morgana.
It's odd that this episode was penned by Jake Michie, one of the show's co-creators, and author of a dozen episodes over the Merlin's five years - but I suppose everybody has their off-days.