See my round-ups of episodes 1, 2 and 3 of series 5 here: Arthur's Bane Part 1, Arthur's Bane Part 2 and The Death Song of Uther Pendragon
We begin with Princess Mithian and her father being taken hostage in their own castle by Morgana and Oden. Arthur killed Oden's son last series (I think) and Oden killed Uther in revenge. Mithian's home/palace/castle is something - I want it. Stately and beautiful without being overly opulent, it makes you wonder why Camelot isn't as nicely decorated. Anyway. Morgana has promised Oden that she will deliver Arthur straight into his hands to do with as he wishes - who wouldn't be tempted by that?
Princess Mithian is sent to Camelot, presumably as bait, and when we get a good look at her aged companion, we see that that is indeed the plan. Her companion is none other than the evil Morgana who has put to use an ageing spell that rivals that of Fred and George. Her role is to ensure that Mithian doesn't 'accidentally' let slip the fact that she's there under duress and she manages with effective creepiness. When Mithian has regained her strength, she spins a believable tale about how her home was captured by Oden and she, her father and Hilda (companion) were forced to flee for Camelot. Her father being the advanced age he is (and Hilda isn't, you think?), they left him behind and now seek the help of Arthur and his Knights to get him back before Oden and his men happen upon him.
It's an impressive performance and Arthur is convinced. As always, Merlin's tingly sense is tingling like crazy but Arthur won't be dissuaded. Merlin has witnessed the interaction between Mithian and Hilda and knows that something isn't right, but can't put his finger on what's wrong. I, for one, can't believe that no one sees through Morgana's disguise. Gwen, too, feels something isn't right and is concerned that Arthur only agreed to help so that he can get his revenge on Oden for killing his father. Arthur assures Gwen that he only wants to help out an ally and they set off the next morning.
Mithian has been warned that terrible consequences will be incurred if she tries to warn Arthur of what is going on. The clever girl manages to get a message to Merlin but Morgana intercepts before the warning bells can be sounded and knocks him out cold. Arthur decides that they'll have to leave Merlin behind with Gaius and Gwaine and Mithian desperately tries to hide her distraught. Gaius knows that Merlin's injury wasn't naturally induced and sending Gwaine off on a fool's errand, does some magic of his own. On his return, Gwaine doesn't even question Merlin's miraculous recovery and they speed off (leaving old Gaius behind) to the rescue.
Arthur enters the cave with Mithian and only Percival for back-up, leaving the others outside to keep watch. Bad, bad mistake. He's perplexed by Mithian's betrayal at first, but then discovers the real reasons behind her deception and doesn't hold a grudge against her. Cue ambush and swordfight and Merlin comes in with a rockfall to save the day, again, much to Arthur's surprise. He also manages to strike Morgana down with magic but it doesn't take a rocket scientist to figure that of course she won't be dead.
It feels blasphemous, but I was a tad bored by this episode. I don't think it added much to the plot and it felt like more of a 'filler' episode in the calm before the storm where bigger things will hit. Also, when will we get more Arthur-Gwen interaction? Now that they're married, they've turned into a boring couple! Where's the romance gone? I'm hugely dissatisfied.