"What are the Vashta Nerada?" Sam inquired curiously. The Doctor put his sonic screwdriver down and gestured to the abnormally dark shadows of the house. "The only thing in the Universe that can strip something to the bone in under a second. They come in swarms; huge swarms. They live on every single planet, including this one, but usually feed on roadkill... I don't know what they were doing, feeding off of humans..." the Doctor said, running his hand through his hair again. Dean slightly lowered his gun. "What do you mean, every single planet? Earth is the only one habituated." Sam insisted. The Doctor laughed. "Oh, of course, humans think that, yes, how did I overlook that?" he chuckled. Dean approached the man warily. "Humans? What does that make you?"
"Well certainly not human, if that's what you mean."
"Then what are you?"
The Winchesters stared the Doctor down with accusing eyes, but he did not budge. "You wouldn't believe me if I told you."
"Funny," Dean said, exchanging a glance with his brother. "that's exactly the sort of thing we tell people. So what are you? A demon? Shape-shifter? Ghoul?" he interrogated. The Doctor looked taken aback. "Nonsense, those things don't exist. But I suppose if you insist..." he took a deep breath. "I'm a Time Lord, from the planet Gallifrey. I'm here investigating the Vashta Nerada, which are also aliens. And I know how to say 'Let's go' in French." he said quickly. Dean looked over at Sam and then back at the Doctor. Suddenly, the two of them burst out laughing. "That is weird! What, you some sort of psycho? Listen, buddy. We've been working as Hunters long enough to figure out that aliens aren't real. So what are you really?" he demanded.
"What do you mean, 'aliens aren't real'? By saying that you're implying I am not real, and you're very lucky I am, otherwise you two probably would be reduced into piles of fat by now. Walking fat. You ever seen walking fat? I didn't think so. I'll tell you, Sam and Dean Winchester. Aliens are very real."
"Don't be stupid, Doctor. The only things that are real are angels and demons and monsters. Aliens simply don't exist."
"Well I could say all those things you just listed aren't real but that would be rude, and I am not rude."
Dean turned to Sam in dismay. "Did you hear that? He's fricking British, Sam."
"If you don't believe me, go into the kitchen and get some meat. Something with a bone. Now!" the Doctor demanded. Sam and his brother jumped, but rushed into the kitchen and tore apart the fridge, searching for meat. At long last, Dean found a raw chicken in the freezer, and brought it to the Doctor, who tore off a leg and looked back at the two. "Watch this and tell me what could possibly do this." he said quietly. With that, he turned and tossed the chicken leg into the shadows. By the time it hit the ground, it was already bare to the bone. Whatever lurked in the shadows had devoured the meat off of it in mere milliseconds. Dean paled. "W-Well, I dunno, something's gotta have been able to do that... we'll just call Bobby. Bobby'll know. Right, Sam?" he said, turning to his brother for support. Sam shrugged, tearing his eyes away from the bone and looking into his brother's eyes. "I... I don't know, Dean. Maybe the Doctor's right. I mean, we deal with monsters and demons every day, so why is it so absurd that there could be aliens?" he said quietly. The Doctor nodded slowly and returned to scanning the shadows with his sonic screwdriver. "Fine. So what do we do?" Dean asked the Doctor. "How do we kill it?"
The alien looked flustered at the question. "Well... we don't. Just stay out of the shadows, not a finger in the shadows. Nothing, unless you want to get eaten." he warned. Sam self-consciously stepped away from the shadows and nearer to the Doctor's flashlight. "If we can't kill it-"
"Them. It's a swarm."
"Okay... if we can't kill them, what do we do?"
"We run."