The blue of the police box was out of place in the dark and damp of Purgatory, its impossible walls seeming to glow through the murk, pulsing with life in a place so devoid of being. Castiel stared at it in wonder, staggering after the mad man who had led him to it. Millennia of existence, and the universe still held wonders for him. Astounding.

"Well, this is me," The Doctor spun on his heel, the police box doors swinging inward at the snap of his fingers. "The TARDIS. I told you about the TARDIS, didn't I?"

"You mentioned it," Castiel said. "Somewhere in the running."

"Ah, yes, running. Love the running! All sorts of nasties down here, spoiling for a good run!" The Doctor smiled through the blood and dirt smeared across his face and straightened his bowtie, bouncing on his heels. Always in motion, that small sad smile fixed on his young face, he was a staggering comparison to the angel with his deadpan look and bedraggled beard, hunched and still as a statue.

"And you, Castiel, you're a runner, aren't you?"

"I've done my share," the angel intoned, his eyes wandering to the TARDIS. His way out, the way to Sam and Dean. He had no way of knowing if Dean had made it out safely; who do angels pray to?

"Yes, yes, of course you have! Lots of running, you and I and those boys of yours!" The Doctor laughed, as though recalling a fond memory. "Few times I didn't think we'd make it, but we always came through in the end!"

Castiel's brow furthered and his eyes darted from the TARDIS to the Doctor. "You've run with Sam and Dean?"

But the Doctor only smiled. "Spoilers."

Castiel's head titled to the side. The Doctor was a strange man. Castiel had thought him human, another soul lost in Purgatory, when he first appeared, driving away Leviathan with a strange gun that emitted airwaves, forcing the Leviathan to dissolve into goo and slither away. Since, he had learned enough to know the Doctor wasn't human, but still he had no idea what he was. And how he talked, as though Castiel should know him personally, although they had never seen each other before now. A strange man indeed.

"Well, I must be going. Places to go, hunters to save." A shadow crossed the Doctor's face and came to rest in his eyes. "But you won't be coming, will you?"

Castiel's eyes shifted downwards. "No, I must stay."

The Doctor nodded. "Just as well. I couldn't take you with me if I wanted to."

The angel looked up, the question in his eyes.

The Doctor fidgeted and stepped closer. "Your escape from Purgatory is a fixed event. For me, it's already happened, and there's nothing I can do to change it and the reaction it has caused. Too many realities are intertwined."

"Explain yourself," Castiel demanded. The Doctor's words made him uncomfortable. Who was this man who so easily treaded space and time, who wandered where even the angels could not follow?

But the Doctor did not explain. He seemed to ignore the question completely. "Ah, yes!" He reached into his coat and held his hand out to Castiel. "Knew I was forgetting something important. Whole reason for this little detour."

"It's a cellular phone," Castiel said, taking it from the Doctor. A small, round flip phone of the bluest blue. TARDIS blue.

The Doctor chuckled. "Observant as always, Castiel." He clapped the angel on the back, turned, and marched to the TARDIS. He'd only just gone inside when he came back out again. "Oh, I am getting old! Forgetting the most important part."

"I thought the phone was the most important part."

"The other most important part," the Doctor corrected himself, stepping close, very close. Castiel could feel his breath, and was sure the Doctor could feel his lack thereof. Castiel remained still as the Doctor placed his hands on either side of the angel's head, but then-an intrusion, a foreign presence in his mind, in his being. The angel reeled back, fully alert, staring at the Doctor in alarm and wonder.

"What are you doing?"

"Oh, come on. It's not the first time we've done this," the Doctor frowned and crossed his arms. "At least that's what you told me when you went into my mind."

"I have never invaded your mind," Castiel insisted. "In the time you've been here, I have not looked into your soul, not even for a second, for fear of what I may find there." And this was true; a fraction of a second was all the angel needed to see the darkness behind that smiling face.

The Doctor was silent a moment, and the two beings merely watched each other with ageless eyes. Then, quietly, "Yes, you have. When we first met. Didn't even ask permission, either, or give me a warning, which is very rude by the way-"

"We are meeting for the first time, now."

"No! Yes! You are meeting me, but I have already met you! Time travel, very confusing, I know, but you will understand, I promise." His face took on a new shade of sincerity. "I promise you, Castiel, you will understand, but right now, I need you to trust me."

Castiel hesitated. The Doctor hadn't given him any reason to doubt him, but allowing a stranger into your mind was not something so casually done.

"What are you going to do?"

The Doctor peered into the angel's eyes, as though trying to see what exactly he was dealing with. "I'm going to construct a wall within your mind, hide away all memory of me. I can't risk anyone knowing I'm involved. Not until I actually become involved."

"And when do you become involved?"

"Oh, not much longer," the Doctor mused. "The wheels are already in motion, now that Dean's out, knocking about upstairs. He'll have found Sam by now, I think. But I really can't tell you about it. Can't tell someone their future. Now, please, let me do this." He reached for Castiel's temples again, but the angel held up his hand. "You know, you could have warned me you were going to be difficult about it."

"I can't know you're telling the truth," Castiel said.

The Doctor sighed and was silent for a moment, staring down at his feet. Then, he looked up again, with a small smile. "Fine, I'm going to cheat. Come here, Cas. I'm going to show you my memories of you and your boys." The angel was still weary. "I won't be going into your mind, merely giving you the option of viewing mine. I suggest you take the chance."

Again, the Doctor brought his hands to the angel's head, and this time Castiel remained still. The Doctor's fingers were cool against his skin; he felt his eyes closing, and a buzzing traveling through him, and then images. He saw Sam and Dean, he saw fire and darkness, he saw monsters and creatures, familiar and strange. He saw himself fighting alongside the hunters and others he didn't recognize, saw them fall to the dark and fire, heard the Doctor cry out. And then, there was space. He was inside the TARDIS-although Castiel had never seen the inside before, he knew where he was. He felt the Doctor's mind pushing him away, warning him to turn back, but he seized the memory. They were in space; they were at war. The TARDIS was hit; the Doctor rushed around the control console, desperately trying to steady the ship, and through his eyes Castiel saw the creature, the Silence, enter, unnoticed in the panic, the confusion. He felt the Doctor's hearts stop, felt his breath catch as the monster raised its hand, as lightning scented the air, as Dean Winchester burned and disintegrated-

"Stay with me, Castiel," the Doctor's voice was soft but stern as the angel tried to pull away, tried to make sense of what he was seeing. "It's alright, it's alright." But it wasn't alright. What was he seeing? What were those things? Why-Dean- "Calm down, Castiel."

And he felt the Doctor's mind inside him again, he felt the wall going up. The Doctor's borrowed memories went first, slipping away, pulled from Castiel's mind as a calmness descended on him and the Doctor spoke.

"Put the phone in your pocket, Castiel. Keep it there, out of sight, but at the back of your mind. You'll remember. When you really need me, when it's time for us to meet, you'll remember. But now, forget me. Forget the TARDIS. Forget all of it."

Castiel's mind became fuzzy, light. He was aware of his memories slipping away, of a blankness taking up the space. Eyes closed, he was aware of time passing, of his own stillness. And then it was gone. He opened his eyes, blinking at the empty Purgatory forest around him. Disoriented, dazed, he didn't know how he had gotten there. His mind was slow and sluggish, waking up. But why had it been asleep? What had happened? Something was missing here. Something important. The last thing he remembered was Dean, escaping, and then the Leviathan . . .

And there was something. Something in the corner of his eye, on the fringe of his mind. Something in his pocket . . . something . . . what was he thinking about? Where was he and how had he gotten there? What was he was feeling, this residue of energy? Something had been here. Something big, something powerful-

And then he sensed them, Leviathan, swiftly approaching. The angel forgot his anxiety and ran, half remembered words drifting back to him.

And you, Castiel, you're a runner, aren't you?

He supposed he was. Everyone was a runner in Purgatory.