Imagine the gang on a hunting trip but they find a sorcerer who of course tries to attack Arthur, but Merlin takes the hit and long story short, Merlin behaves like a dragon for 24 hours.
He doesn't breathe fire or anything (yet), but he doesn't talk and he seems pretty fond of anything golden or shiny. And I mean ANYTHING.
They bring him back to Camelot and they lock him in his room for, well, obvious reasons. Gaius along with Gwaine look after him and Gwaine gives him a coin from time to time to keep him busy. Merlin keeps all his "treasures" on his bed and no one is allowed to come near it.
Then Arthur comes in to check up on his servant and Merlin, who was organising his (Gwaine's) coins just stills and seems mesmerized by the prince's hair. Arthur tries to ignore it and listen to what Gaius is saying, when suddenly his servant starts petting him, carding his fingers through blond locks.
For a moment everyone is silent and then Merlin just tries to drag Arthur towards his pile of "treasures" while Gwaine is dying of laughter in the backgound.