Merlin returns to BBC One for a fourth series next month with arguably its scariest and most affecting episode to date.

The story picks up twelve months after the dramatic events we saw towards the end of last year’s run. Uther Pendragon (Anthony Head), still heartbroken from Morgana’s betrayal, hardly leaves his chambers, and in his wake Arthur (Bradley James) – assisted by his uncle Agravaine (Nathaniel Parker) and the newly formed knights of the round table – has taken charge of Camelot.

Having been banished in the last series, Morgana (Katie Mcgrath) travels to the isle of the blessed, where she manages to tear open the veil between the worlds of the living and the dead, plunging Camelot into darkness and despair. We won’t elaborate further on the storyline of this opening episode, but suffice it to say that this is easily the scariest and the darkest the show has ever been.

That’s not to say that the shows idiosyncratic comical aspects are no longer present. Indeed the episode is surprisingly funny considering its themes, with the banter between Arthur and Merlin remaining as humorous as ever. But let’s be clear; this is no romp.

One scene in particular, where Arthur explores a village that has been attacked by the Dorocha, that serve as the story’s primary antagonists, is easily the most frightening, suspense-laden sequence that the show has ever produced. The visual representation of the Dorocha doesn’t quite live up to the tension produced by their slow, moody reveal, but they’re certainly as terrifying as an enemy could possibly be in a show that’s meant to accommodate younger viewers.

In fact, the Dorocha show so much promise as a villain that it’s a shame that they seem a little underdeveloped in this opening story. They only really do one thing throughout the story, which, though scary at first, begins to lose its effectiveness towards the episode’s conclusion. Hopefully this is a problem that will be remedied in the concluding instalment of this two-parter.

Whereas it could be argued that the first three years of Merlin almost followed a procedural format – with someone new being invited into the castle every week who would inevitably turn out to be an evil sorcerer intent on killing Uther and/or Arthur, only to be thwarted by the combined efforts of Merlin and Gaius in the episode’s closing moments – Series 4 seems set to depart from that formula in a number of ways.

The first and most noticeable change is the addition of the knights of the round table to the principal cast. Most of the knights don’t really have enough screen-time to make an impression in this first episode, but the addition of Sir Lancelot (Santiago Cabrera), one of the only other characters that is aware that Merlin possesses magical abilities, allows for some humorous moments in this otherwise bleak opening.

Another major change to the Merlin formula is the reduction of Uther’s role in the story. Anthony Head literally only has one line in the entire episode, which is heartbreakingly delivered by the former Buffy actor, but one can’t help but think that Uther Pendragon’s time on the show is drawing to a close.

In another departure for the show, Morgana (Katie McGrath) is now an out-and-out bona fide villainess. McGrath talked in the Q&A session that followed the screening about how she wants viewers to be able to understand Morgana’s logic, and her reasons for turning against her former friends. This doesn’t really come across so far, however, and we only hope the show’s writers give McGrath the material necessary to make this connection with the audience in upcoming episodes.

Strong, if minimal, performances are also delivered by Angel Coulby as Gwen and Richard Wilson as Gaius, with newcomer Nathaniel Parker failing to make much of an impression as Arthur’s uncle Agravaine, though it seems likely that he’ll play a more pivotal role in future stories.

The opening episode of Merlin’s fourth series is a solid, accomplished piece of television. It doesn’t take many risks or break any boundaries, and is unlikely to win over the show’s detractors, but fans on the other hand, are in for a real treat.

Airs at 7.50pm on Saturday 1st October 2011 on BBC One.

> Order the Series 4 Volume 1 DVD on Amazon.

What did you think of the episode? Let us know below…