Understanding
"I'm afraid I won't be coming with you. Not this time. I'm sorry. I have an urgent errand to run for Gaius. Vital supplies that I can't attain here." For once, Merlin's excuse was somewhat plausible. The aftermath of the battle between Camelot and Morgana's army would require the use of herbs Gaius wouldn't normally use.
"Vital supplies?" Unfortunately, Arthur's doubtful and hurt expression betrayed his disbelief.
"Yes, it's—"
"I don't understand," Arthur cut him off before he could further develop the lie. "I know you, Merlin. You have knowingly drunk poison, accompanied me to fight a dragon, and maintained courage in the face of two immortal armies. You're not a coward. In fact, you're probably the bravest man I've ever met. So what makes this time different?" The king's eyes bored into Merlin's. "Please don't lie to me."
The servant's shoulders fell, and he closed his eyes as he mentally scrambled for a response. He couldn't tell Arthur about his magic—he had personally put the final nail in that coffin—but he owed Arthur the truth.
When he opened his eyes, Merlin found Arthur not glaring at him like he had expected but instead staring at him worriedly, and he smiled slightly. Pride flooded him at the thought that this man, this king, was his best friend.
"I can't tell you. But," he hurried when Arthur opened his mouth to protest, "you will find out when the time is right. I promise."
The king searched his servant's face for a moment before nodding. He was obviously displeased with the answer but had concluded that he wouldn't get any more information out of Merlin at the moment.
Arthur held his hand out. "I'll hold you to that, so you'd better stay safe until I get my explanation."
Merlin grasped his friend's wrist and let Arthur grasp his in return. "The same to you, sire."
A/N: I wrote this out of anger in response to the original scene in the show. It bothers me a lot, because it's like Arthur has completely forgotten everything that he has seen Merlin do, including things that were probably just as frightening as facing Morgana's army. He should have known that something was wrong and that it wasn't just Merlin being scared. The writers took Arthur's supposed obliviousness to a level that was beyond what I could believe.
