Gwen: One thing I know for certain. That whoever did this lives among us. Whoever did this has betrayed us all.
[She turns to look at the knights then back again.]
Someone who is free to roam the Citadel as they please, someone who has access to the King's stables...
[Merlin thinks he can tell where this is going.]
the King's prison, even the King's food. There is only one I know of…
[She turns around and looks at him.]
Merlin.
[He stares at her.]
[KING'S PALACE- DUNGEONS]
[Merlin is thrown in a cell.]
Taken from the Merlin wikia.
Merlin made a sort of choking noise.
He couldn't help it, and he wished he could stop. The noise continuing to escape his mouth without permission was somewhere between a laugh and a sob, and he didn't know how else he could react to the accusation. That Gwen - sweet Gwen the serving girl, who was kind to those who didn't deserve it and liked to carry a heavy pitcher of water with her - was standing before him and accusing him of murder was surreal.
The knights had turned to watch him. Gwaine looked concerned and wary, Mordred's face was shocked and open, Leon seemed uneasy, and Percival betrayed no expression. He forced himself to stand upright and face Gaius, who looked grim and older than Merlin had seen him. He had likely seen other friends framed during Uther's purge of magic, and there was something in the way he stood that told Merlin that this would not end well.
He didn't need Gaius to tell him that. He had figured that much out for himself.
Merlin, standing up straight and pushing his emotions into a cupboard in the back of his mind, managed to stop the laughter. He met Gwen's gaze, which was somehow cold and calculating, but the moment she saw suspicion in his features, it shifted to distraught at once.
"It brings me no pleasure to suspect one of my oldest friends," she half-sobbed, moving around the bed to grasp Arthur's pale, limp hand, "but there is no other who had the opportunity except myself, and I would never do such a thing to my husband."
"Milady," he addressed her in a deliberate acknowledgement of the rift between them that now may never be mended, surprised at the angry tone of his own voice, "you and I were the only two with the means to poison him, and I would walk barefoot into hell for Arthur."
Gwaine stared at him like he'd never seen him before, and if the look on Leon's and Gaius's faces were anything to go by, he was doing an unintentional but decent job of intimidating them all. Percival's expression was guarded, but the look on Mordred's face was of outright fear. Arthur breathed, and Merlin watched the slight rise and fall of his chest and hoped that all was not lost.
"I'm going to try to forget that you just said that," Gwen said tearfully, looking absolutely heartbroken. "I know Arthur wouldn't want accusations thrown around like this, and -"
"No, he wouldn't," Merlin practically snarled at her. "He wouldn't want accusations thrown around the people he trusted. Who would have the most reason to poison him? Who wants the throne?"
"Someone who was a trusted member of this court but turned out to be passing secrets and plotting assassinations," Gwen cried. The hand not clasping her husband's went to her breast, and he suddenly remembered Morgana making the same motion when she was nervous and had some malicious secret to hide.
Merlin's temper rose in the way she had likely meant it to. "I am not Morgana!" he shouted, making everyone jump. Mordred, especially, looked terrified, and Leon seemed particularly startled. "I am not Agravaine! I have never conspired against the kingdom and I have never committed treason against the king! I don't know what you think you're doing, but I have never been anything but loyal! If I didn't know any better, I would accuse you of covering up for -"
"Enough!" Gwen shrieked, the tears in her eyes starting to roll down her cheeks, and there wasn't a single other sound in the room. "Merlin, you leave me no choice!"
"Gwen," Gwaine interrupted the start of another shout, which was frustrating, even if it was for the best. "I know that this past month has not been easy for anyone, least of all you, but Merlin would never harm Arthur."
She shook her head, closing her eyes, swallowing, and opening them again. "And he would never slander me in such a way either," Gwen sobbed. She took a deep breath, composed herself, and looked to the door. "Guards?"
"Guards?" Gwaine repeated, shaking his head and looking aghast. Two guards entered the room and awaited instructions. "He has done nothing!"
Merlin understood with a clarity that overwhelmed him at first. All the small things that had been so off about her since her rescue from Morgana's trap and Elyan's death had not been because she was grieving. There had been weeks of sly looks and diverting Arthur's attention as she did something and whispers of another traitor in the court.
He saw Gaius watching the scene with a solemnity that he had acquired after years of standing by and letting things happen. They locked eyes, and in a moment of eye contact, said all that need to be said.
"He sounds as if he is enchanted," Gwen argued, and though Merlin hated to, he blinked and turned calmer eyes on her.
"You sound as if you are," he replied.
They each watched the other with no attempts to hide their true emotions. Gwen's face was cold and imperious, almost afraid that someone had found her out but sure he wouldn't be believed. Merlin glared at her with revulsion and pity for what Morgana had done to her. She squeezed Arthur's hand in what might have been a portrayal of grief to others, but he recognised it for the subtle gloating it was.
"Guards," she repeated, and took her hand from her heart to point at Merlin. Her finger was steady. "Arrest him."
He stared at Arthur's face as they grabbed his shoulders roughly, trying to memorise the curve of his nose and the way he slept with his eyes half-open. If Gwen, who was so clearly enchanted he couldn't believe he hadn't seen it before, managed to convince them that he was guilty, and if he couldn't sneak out to save him, he might not be able to attend Arthur's funeral.
He glanced around the room and realised that it may not be hard for her to convince others of his guilt. Gaius watched gravely and Mordred helplessly, but the rest of the knights watching had obvious doubts. Even Gwaine, who had polished boots and been chased through castles with him, was staring at him as if he'd never seen him before.
The guards shoved him out of the room, and his last sight of Arthur was blocked by Gwen, who stood at the foot of the bed and smiled to herself.
.
Honestly, though, what happened there? I was asked by parimalik to write something about the knights in that episode, and as I rewatched it, I kept thinking that there was no way any of the knights (except maybe Percival, because we know nothing about his motivations beyond "my enemies are your enemies" and that doesn't equal trustworthiness) would simply allow a friend to be thrown in the dungeons without the medieval equivalent of a smoking gun. Gwaine is too loyal to Merlin, Mordred wants to be on Merlin's good side, Leon has known him from the start and I'm pretty sure he's seen or heard about Merlin's many attempts to actively save Arthur's life, and Percival, who we know nothing about, has probably seen enough of how things work in Camelot to know that Merlin is innocent. So why did they allow him to be thrown in the dungeons?
This is the only thing I could imagine happening in that small gap between scenes.