When a terrorist escapes custody during a routine handover, Will Holloway must team with disgraced MI5 Intelligence Chief Harry Pearce to track him down before an imminent terrorist attack on London.

"The mythical city of Camelot, in a time before history began. A fantastical realm of legendary beasts and mysterious peoples. A dangerous world in which magic has been banned by the ruthless tyrant. Uther Pendragon. When Merlin, a young man gifted with extraordinary magical powers, arrives in the kingdom he quickly makes enemies including the heir to Uther's crown, the headstrong Prince Arthur. But guided by Uther's wise physician. Gaius, Merlin...

Swords, sorcery and swashbuckling adventure return in this third series of the hit BBC series, following our heroes Merlin and Arthur as they face their greatest challenge yet. When misbegotten secrets are exposed and old friends choose new sides they soon discover that the great battle for Camelot is only just beginning.