No Harm Will Come to You Here (слэш)

"That was magic," Arthur accuses.
"Oh." Merlin just nods, looking unperturbed.
Arthur fights to keep his expression calm, mind racing and heart pounding. "Sorcery is outlawed in Camelot. On pain of death."
Merlin splutters, finally fixing Arthur with an affronted glare. "It's not like I meant to do it!"
***
Merlin gets hit with a spell meant for Arthur and loses his memory. Revelations ensue. Set sometime after 5x02.