Merlin escapes from Morgana before she can heal him or implant the Fomorroh, leaving him lost and injured in the middle of the forest. By the time Arthur and Gwaine find him he is sick and exhausted, but on the way back to Camelot he manages reveal something shocking - he knows who the traitor in Camelot is, and that they are in league with Morgana.But will Arthur believe him? And what will he do with this knowledge?
or
An alternative plot for s4e6 - A Servant of Two Masters, where Merlin doesn't get healed or brainwashed and remembers Agravaine brought him to Morgana. Also the classic magic confession and Merlin being our favourite self-sacrificing idiot.Pre-slash but gen if you squint
Merlin was sharpening one of Arthur's swords out on the training field when a new alpha knight decides to try and boss him around and then try to teach him some manner. This does not sit well with Arthur.
Arthur yells at Merlin, pushing him away in the process. When the unexpected strikes, he's faced with the possibility of them being the last words he ever spoke to his best friend.
AKA, Merlin gets bonked on the head and Arthur panics.