Tipped off by Agravaine of their journey, Morgana sends mercenaries to attack Arthur's party, and Merlin, separated from the others, gets captured. Before releasing him, Morgana infects him with the head of a Homorroh (hydra) which makes him want to kill Arthur. Gaius, noting Merlin's woefully out of whack behaviour, realises what has happened, but the condition proves difficult to break and requires Merlin, during only a few hours of mental clarity, to turn himself into his old Emrys visage, infiltrate Morgana's lair, and destroy the Fomorroh mother beast. Meanwhile, Agravaine, aware that Arthur suspects his treachery, tries to implicate Gaius.
—don @ minifie-1