In an attempt to escape, Arthur leads them into the Valley of the Fallen Kings, a place steeped in magic, but is shot and gravely wounded.

Even Merlin’s magic is not strong enough to save him, but a mysterious stranger by the name of Taliesin (Karl Johnson) uses a powerful spell to cure Arthur completely. He then takes Merlin into the Crystal Cave, where he tells the young wizard that he has been brought to the cave for a purpose. Here, Merlin looks at the crystals and sees an horrific vision of Morgana stabbing Uther with an ornate dagger.

When they get back to Camelot, Merlin tells Gaius about what he saw, but Gaius is sceptical and tells Merlin to be careful. Soon, however, Merlin is seeing events that he has previously seen in his vision. When Arthur says that his present to Morgana at her birthday feast will be (yes, you guessed it) a dagger, the stage is set for events that will shake Camelot to its foundations.

Throughout the show’s first two series, Merlin’s stories were well written but fairly safe retellings of the Arthurian legends, greatly enlivened by the humour of the two leads. Series 3, though only five episodes in, is showing a new confidence and a willingness to move away from that formula, a move that’s making it every bit as essential to Saturday nights as the more established Doctor Who.

Following the broad comedy of ‘Goblin’s Gold’, ‘The Crystal Cave’ pitches the audience straight into gothic horror. As the series unfolds, Morgana is becoming more manipulative and increasingly afraid of Merlin and what he might do to her. Chilling and coquettish in equal measure, this is definitely Katie McGrath’s story, drawing the viewer in almost magically as the increasingly duplicitous and evil Morgana.

Colin Morgan once again gives a strong performance as the conflicted sorcerer, leaving the humour behind for all but the first few scenes. In fact, this is probably the show’s most mature episode to date in terms of storyline and acting. The new kid on the block is now very definitely a confident adult striding on to possible greatness.

Airs at 7.40pm on Saturday 9th October 2010 on BBC One.