Three hours passed in a slow, cold sweat for Sam. He stared at his computer screen, ignoring everyone around him, trying to figure out when they could've grabbed Dean and replaced him with a shifter. When he was checking out the supposedly haunted house with the others? Or had another shifter snuck into the TARDIS while Moriarty was keeping him occupied?

Or… Sam rubbed his neck. What if the Dean he'd rescued from the demons was a shifter?

Maybe he was making it into too big of a deal. Maybe he was just desperate enough to find Mycroft that turning Baby into a time-travel machine wasn't an obstacle.

Besides, he hadn't noticed Dean acting weirdly until today. So if it was a shifter…he would've had to jump Dean when he was going out to the TARDIS earlier for a nap.

"Hey." Amy touched his shoulder.

Sam jumped.

"Wow, nervous much?" Amy sat down beside him, smiling.

Sam forced a grin. "No, you just startled me. Did I miss anything?"

"Not unless you count the Doctor pacing around like a caged animal and Sherlock staring at the wall and refusing to talk to anyone." She picked at a hangnail for a second, then said, "We're close, aren't we?"

Close to Mycroft, maybe. But still no closer to an idea on how to defeat him. Or why he thinks my psychic crap is important.

"Hey guys!" Ash barrelled into the room, carrying his laptop. "Guys, I think-"

He slammed into River and Dean, who had chosen that moment to materialize in the middle of the floor. Dean and River barely moved, while Ash bounced off of them and onto his back, clutching his laptop to his chest.

"Ash! Sorry, man." Dean crouched down to help Ash up.

Sam saw the flash of a knife. "No!" he yelled, lunging up from his chair. Without even thinking, he swung his hand.

Dean and Ash were knocked apart by an invisible force. Ash landed on his back again, but Dean slammed into the far wall of the room. Sam stepped forward, hand outstretched toward the shifter masquerading as his brother. He could feel a headache blooming in the back of his head, but he shoved the pain away. He stepped forward quickly, until he stood between the shifter and everyone else in the room.

Just in time. The power dropped away, leaving Sam light-headed. He took an uneven step backward, and shifter-Dean lunged, the knife in his hand glittering as he swung. Sam dodged the knife and smashed his elbow into the shifter's face, wincing as it grunted with his brother's voice. A fist smashed into his face, spinning him to the side, and the shifter followed up with a kick at his legs.

Sam hit the ground, yelling the faint headache hit full force, making his entire skull throb. He rolled to his side, felt Amy grab his arm. He tried to jerk free.

"It's okay!" she was yelling at him. "Sam, it's okay, they've got him!"

Sam looked up,wiping blood from his face. Sherlock, the Doctor, and Ellen had all jumped into the fight, grabbing the shifter and pulling him away.

"Dean Winchester, what got into your cotton-pickin' mind!" Ellen yelled at him.

Sam staggered up to his feet, reaching for the silver knife at his hip.

The shifter grinned at him through blood-stained teeth. "Come on, psychic boy. Do it. Shove that knife into your brother dear's heart. Or are you not man enough to kill your brother?"

Sam dug his fingernails into his free hand. "You're not my brother."

"Sam, wait-" the Doctor started.

Sam slammed the knife into the shifter's gut.

Shifter-Dean howled and collapsed to his knees, flesh starting to melt. A swarm of shadows burst from the shifter's skin, buzzing angrily as the shifter's bones clattered to the floor.

"Light!" The Doctor yelled, jumping in front of Sam and brandishing his sonic screwdriver. "Ellen, Jo, get flashlights, now!"

The Vashta Nerada swarm roiled in the air in front of Sam and the Doctor, the rasping buzz making Sam's skin crawl. It tried to dart to the side, but the Doctor followed it, poking at it with his sonic screwdriver.

"Ha! Take that, you shadows!" the Doctor shouted, poking again. "Take my friends, will you! Haha, I'd like to see you try!"

The buzzing shifted in tone, sounding even angrier than before.

River sighed as she stepped up to the Doctor's side, pulling her own sonic screwdriver from a pocket. "You had to antagonize it, didn't you?"

Ellen and Jo burst from a storage, carrying several heavy-duty flashlights and camping lanterns. As River and the Doctor struggled to keep the Vashta Nerada away from Sam, everyone else quickly set up the lanterns, trapping the Vashta Nerada in a ring of light.

Finally the Doctor sighed and dropped his arm. The Vashta Nerada swarmed in its prison, bouncing from one beam of light to another, and Sam swore he could almost hear it shrieking in his head.

"Okay," Amy muttered, taking a deep breath. "Shifter. Dean was a shifter."

"Which explains how John and the TARDIS were captured," Sherlock said. "But how long was he a shifter, do you think? And who else here could be a shifter?"

Everyone immediately eyed everyone else with suspicion.

Sam groaned. Not again. As he explained to River about shifters, he passed his silver knife around the group. No one's skin sizzled at contact with the metal. No one's eyes glinted golden.

"Can you hear this swarm?" Sherlock asked in a low voice as he handed back Sam's knife. "Like you did the first time in the vampire nest?"

Sam shook his head. "I thought I could right at first, but there's nothing now."

"And none of us have ever heard anything." Sherlock tapped his chin. "I think I know why the Shadow wants you. You seem to be the only one whose mind is on the same mental frequency of the Vashta Nerada, which means that, theoritically, you could communicate with them. I wonder how far away it would reach."

"So?" River jumped in. "The Shadow has to have some way of communicating with the other swarms. Otherwise he couldn't get them to inhabit a shifter and imitate a human, right Doctor?"

The Doctor coughed. "Right."

Sherlock sent River an irritated glance. "Yes, but don't you find it odd that the shifter swarm imitating Dean suddenly burst out from its body, almost as if it had no control over itself?" He spun around to meet the Doctor's eyes. "You kept talking about how this isn't how Vashta Nerada behave, that they don't display this sort of patience. But what if the Shadow is a new level of Vashta Nerada? One who is patient. One who can plot and plan and manipulate all his pieces on the board. We know he controls the other Vashta Nerada somehow. But how?

"And this is where Sam's psychic ability comes in. He's the only one of us who can hear the Vashta Nerada speak when they've not inhabiting a body. His mind can can communicate with theirs. Therefore, I think it's safe to conclude that the Shadow wants Sam because he believes Sam's psychic ability will let him control the other Vashta Nerada even better-no matter how far away he is, in any sort of circumstance."

Everyone was silent, processing, their minds catching up with Sherlock's.

Sherlock lowered his voice. "Imagine what the Shadow could do with a mind like Sam's."

Sam clenched his fists.

The Doctor's face twisted. "He's not going to take over Earth. Not on my watch."

"Not on any of our watches, Doctor," Ellen said.

Rather than argue, as Sam half-expected, the Doctor just gave her a sharp nod. He turned away from the group, but not before Sam caught a look of deep, profound sadness pass like a shadow over the Timelord's face. River gently put her hand on the Doctor's shoulder and squeezed.

"So…he has Rory, and John, and Dean, to stop us from stopping him. From taking Sam," Amy said.

"To stop me from fighting any more," Sam said heavily.

Everyone gave him startled glances, and Sam just shrugged, dropping his gaze to the floor, not wanting to see the pain in their eyes. It was too much like how Dean looked at him these days.

"Sam-" the Doctor started.

The sharp ring of a phone cut through the air like a whip.