Story details for the eighth episode of Merlin Series 5 have been revealed.

When Merlin agrees to help a young Druid boy, he has no choice but to leave Camelot on a dangerous mission.

And with the citadel distracted by the visit of the fearsome Sarrum of Amata, it seems that no one will notice his absence.

But as the sky darkens, so do Merlin’s suspicions – is there more to Daegal than meets the eye? And can a man as ruthless as the Sarrum ever really be trusted? Too late, the young warlock realises he’s made a terrible mistake.

Colin Morgan is Merlin, Bradley James is Arthur, Angel Coulby is Gwen, Katie McGrath is Morgana, Richard Wilson is Gaius and John Hurt is the voice of the Great Dragon. Tom Hopper, Rupert Young, Eoin Macken and Adetomiwa Edun return as The Knights of the Round Table. Special guest stars include John Shrapnel as Sarrum, Matthew Clancy as Albin and Mitch Benn as Gerrin.

Episode 8 airs Saturday 24 November 2012 on BBC1.