After the events of A Servant of Two Masters, resulting in a confrontation with ‘old Merlin’, Morgana now fears her mortal enemy more than ever.

Running with the assumption that the old wizard must be receiving information from inside Camelot, and with some prompting, she makes Gaius her next target. In order to locate ‘Emrys’, Morgana enlists the services of a warrior priest who has a reputation for getting at the truth.

Meanwhile, devious Uncle Agravaine seeks to divert attention away from his own dark allegiance. Worked into a corner, Arthur seeks proof and is forced to assent to a ‘little chat’ with Gaius in order to gauge the old man’s loyalty…

This week’s story follows on a logical path from the last, save for the odd fact that Merlin now obviously knows the location of Morgana’s lair. Why does he not just gather up the Knights and head out to finish her off we wonder? Putting that aside, this episode is all about misconceptions and loyalties. As ever, the principals all fail to see Merlin for what he is, leading to a whole host of trouble for Gaius.

The aged medic’s interrogation by Agravaine, with a troubled Arthur watching on, is a joy to watch, hinging as it does on the events surrounding Uther’s death. It is great to see Nathaniel Parker so well used as he playfully dances around the issue of magic, hoping to get Gaius to implicate himself.

The guest star this week is Scots actor Gary Lewis, better known to a generation of filmgoers as Billy Elliot’s dad. Here, Lewis gives us Alathor, a brooding intense warrior priest who’s plainly a man you don’t refuse to answer. A muscular half-naked manservant, for those who enjoy such things, also accompanies him at all times.

With flaming torches and spooky tunnels, there is some action too as Merlin finds himself friendless and having to do some detective work. He does gain help from an unexpected quarter though. We’re pleased to report there are minimal amounts of flying backwards this week, though that’s not to say we go without magical confrontation.

Placed at the heart of the action for once, Richard Wilson gives a defiant performance and the episode clearly belongs to him. Like Gwen, Gaius has been somewhat underused this year in amongst all the Knights and matters of kingship. Here he finds his loyalties tested to the limit, with a result that is quite surprising.

The Secret Sharer is another solid, enjoyable episode with building tensions both inside and outside the walls of Camelot. Great stuff!

Airs at 8.20pm on Saturday 12th November 2011 on BBC One.

> Order the Series 4 Volume 1 DVD on Amazon.

What do you think of Series 4 so far? Let us know below…