Merlin, Guinevere, and Gaius assist Arthur as Camelot enters difficult times. With Morgana's powers growing rapidly outside Camelot, Merlin must be more cautious and vigilant to protect Arthur and prevent Camelot's destruction. Agravaine, Arthur's uncle, becomes Arthur's trusted advisor. Prince Arthur takes charge and later becomes king after his father's death. Plot Ī year has passed since Morgana betrayed Uther, who has been left heartbroken.

Notable guest actors include Santiago Cabrera, Emilia Fox, Gemma Jones, Phil Davis, James Callis, Lindsay Duncan, Gary Lewis, Charlene McKenna, Janet Montgomery, Terence Maynard, Ben Daniels, Miranda Raison and Caroline Faber. Supporting cast members include Adetomiwa Edun, Eoin Macken, Tom Hopper, and Rupert Young. Nathaniel Parker joined the regular cast in this series (credited as such for the first time in 'Aithusa') and John Hurt returned as the voice of the Great Dragon.

Credited as a regular, Anthony Head left the series after his character was killed off in 'The Wicked Day'. Series four stars the regular cast from the previous series including Colin Morgan, Angel Coulby, Bradley James, Katie McGrath, and Richard Wilson. Writers include Julian Jones (head writer), Howard Overman, Jake Michie, Lucy Watkins, and Richard McBrien. The directors of the series include Alice Troughton, Alex Pillai, Justin Molotnikov and Jeremy Webb. The series producer was Sara Hamill, and executive producers were Johnny Capps and Julian Murphy. It consists of 13 episodes originally shown on Saturday evenings on BBC One and BBC One HD (repeats shown on BBC Three).

The fourth series of the British drama series Merlin began on 1 October 2011 with the episode 'The Darkest Hour - Part 1'.

Merlin season 3 episode 4 full episode