Beckett pushed the window up with gloved hands. It opened easily, despite the age of the building and the creaking of the frame. Someone had opened it recently and, strangely, taken the time to close it behind them. She bent down and climbed through it carefully, trying not to disturb the smudge of dried blood on the sill.

"Call CSU," she instructed her colleagues. "This is dry. Whoever climbed out here is long gone, but this might show us where they've taken Castle."

She followed the trail down the rickety, twisting steps. More of the rock salt from the apartment was scattered erratically over the fire escape, on the landings, steps and railings, like someone had been covered in the stuff and running. A series of small blood droplets ran down the left side of the steps, some smudging on the railings. Probably the person who had been injured inside. It was an interesting pattern, almost like the person had been carried. Except running down this fire escape carrying a person with only a mild cut who didn't want to go seemed almost impossible. Three floors from the ground, where the stairs turned in on themselves, the railings were bent out impossibly. There were new scratches in the paint job that had been applied in hopes of hiding the rust. One of the thick metal bars was broken across the middle. Beckett crouched to examine it. It was the strangest thing. She could hardly imagine how much strength it would take to break the bar like that. It was a new bar, presumably replaced when the paint job was carried out. Solid metal, broken. Not cut. Not torn off at a join. Twisted, shoved out and broken. From the turn in the stairs, a much thicker trail of blood led down the steps beside the droplets. Beckett breathed deeply and tried to ignore the possibility that it was Castle's.

Beckett jumped from the ladder at the bottom of the fire escape, landing lightly in a clear spot amongst the trash and broken glass that littered the ground. Ryan and Esposito were waiting for her beside a trio of dented and knocked over trash cans. They had gone down the stairs to avoid further disruption to the evidence.

"CSU are on their way," Esposito informed her. "Any sign of him?"

"There's been a struggle," Beckett told them. She described the scene three floors up. "It almost looks like it's not humanly possible."

Ryan and Esposito exchanged a glance at that.

"What?" she asked.

Ryan hesitated.

"Do you know how someone can rip a solid iron bar in half?"

"There's this book series… hey, what's that?" Ryan frowned, stepping forward to examine the wall. "Does this look deliberate to you?"

Beckett and Esposito crowded in to look. The blood smear on the wall formed a definite circle with a strange design inside it. Underneath it was written: 3+ D 1C.

Ryan and Esposito exchanged another look.

Beckett looked pointedly at them. "Any idea what this means?"

Esposito cleared his throat. "Well, I don't know about the numbers, but I'm pretty sure that's a devil's trap."

"A devil's trap?"

Ryan picked up the story. "Those books I mentioned? They describe this design in detail. It's for capturing demons. If a demon steps inside one, it can't get out, basically, so it can't kill you while you exorcise it. Although this one's kind of small and on a wall, so I don't know what kind of demons they were trying to catch…"

"So you think this could be someone suffering from some kind of delusion that the books are real?" Beckett asked. Sometimes you had to move Ryan along before he lost track of what he was talking about and started turning into Castle.

Esposito answered for his partner. "The books are about brothers called Dean and Sam, who fight evil. Demons have super strength. There are plenty of descriptions of scenes like the one on the fire escape."

"So you're saying someone's decided that Dean and Sam Winchester are the Dean and Sam in the books and have kidnapped Dean to… what? Help him out? And Castle just got in the way so they took him too?"

"Or Dean and Sam Winchester are Dean and Sam," Ryan blurted, before looking away and pretending he hadn't said anything.

"Okay… moving on. If someone is having delusions based on the books, where would they take Dean and Castle?" Beckett snapped a picture of the symbol and the letters beneath it with her phone, before waving over a CSU tech who had just ventured around the side of the building after arriving out front.

"That depends," Esposito told her as they moved out of the way of the tech. "If they think they're helping, they probably live in a place a lot like Frank Walter's apartment. Maybe even have a wall full of weapons and a series of fake IDs. If they're demons they could be anyone. They look just like ordinary people, but their eyes turn black when they're being especially demonic."

"Or yellow," Ryan interjected.

"Or red if they're a crossroads demon. Yeah, this black smoke takes over the person's body and the demon steals their life and goes around doing evil…stuff. So it could be anyone."

"Only they usually seem to do stuff at night, and they would probably want somewhere where no-one can hear screaming or no-one cares."

"Wow," said Beckett, "You guys are never giving me crap about liking Castle's books again." She examined the ground closely. No tread marks. "No sign of a car, but at least one of them was bleeding badly. He wouldn't get far. The blood trail runs out here, but there's no way they could have got a car down here without leaving some kind of sign." She pointed to a small pool of blood in the centre of the alleyway.

"Detective Beckett?" The CSU tech spoke up. She'd moved away from the blood drawing and was crouching beside the chain link fence at the dead end of the alley. "Come and look at this."

Beckett approached, Ryan and Esposito following her. The tech pointed at something with her tweezers. It was a piece of red and grey plaid flannel, perfectly ripped to form an isosceles triangle that pointed in the direction of the fence. Beckett's heart lifted. That seemed like something Castle would do. Leave them an arrow. And he did wear plaid shirts sometimes. She didn't recognise this particular one, but still, it gave her hope. Even if it wasn't Castle's, it meant that at least one prisoner was still in good enough condition after the fight to leave clues for them.

Beckett climbed the somewhat shaky fence. It was bent a little at the top, but not so much it was obvious anyone had climbed it recently. The alleyway was narrow and dirty, old buildings rising six floors up on either side, blocking what little light was making it through the heavy cloud. Whoever had been leaking the tiny drops of blood had either stopped bleeding by the time they were over the fence, or the filth in the alley hid it. Beckett was beginning to think they had read too much into the small piece of cloth by the time they reached the exit. They came out onto a wider street. Shopkeepers were just pushing up metal roller-doors to open for business, and few other people were about. Old cars, some broken down or abandoned, others cared for but wearing out sat parked at intervals along the street. There were no tread marks, no blood trail, no salt.

Then Ryan spotted it: a few yards down the street, on the hood of an early eighties Ford, a red and grey triangle pointed to the right.

Beckett was just calling it in to CSU when the commotion started. A crashing and thumping came from a building across the street. It was empty and dark, the windows boarded up, but padlock on the metal roller swung haphazardly from its broken loop and the door bounced slightly up and down. Beckett shivered as a sudden shower of cold rain came down, and tied her hair up as the wind whipped it into her face. She signalled to Ryan and Esposito, her hand moving to her weapon, and approached the door.

"NYPD! Open up," she called.

There was no reply, but the thumping and crashing continued, an indistinct yelling accompanying it.

Esposito edged the roller-door up slowly and Beckett nudged the main door open slightly, checking inside.

It was dark, but four figures were clearly visible in the dark. A woman and three men, fighting tooth and nail. The biggest one had a second man against the wall. Beckett could just make out the silhouette of Sam Winchester's knife as they struggled with it. They were yelling at each other, threatening words that Beckett couldn't make out over the rain on the metal of the door.

But it was the other ones who scared her. The strange man who'd showed up behind her in Frank Walter's apartment was in motion, his coat floating behind him like wings as he ducked out of the woman's grasp and threw her hard against the wall. Thunder boomed just as she hit it, making the room seem to shudder.

"Where is he?" Cas growled. It was low and quiet, but strangely seemed to penetrate all the surrounding noise.

Beckett was just raising her weapon to try to stop the fighting when the woman stood up, wiping blood from the corner of her mouth and smiling.

"Lost your boyfriend, Angel?" The woman mocked.

Cas seemed to fly across the room by himself. He hit the floor with a thud three feet away from Beckett and rolled to his feet immediately, coming up with a long knife, seemingly out of nowhere. Actually, it was more like a short sword, silver-white and flashing in the dark room.

Heavy raindrops battered the roof. Beckett edged into the room, gun held ready. "Everybody stop!" She yelled, but her voice was drowned in another roll of thunder.

Cas lifted the woman up with his free hand and tossed her against the wall again. "What have you done with Dean?"

A flash of lightning illuminated the room. Beckett's eyes widened in shock. The woman slowly climbing to her feet was the spitting image of the sketch she and Castle had been showing around the previous afternoon. She was Frank Walter's wife.

Behind her, Ryan and Esposito were trying in vain to separate Sam Winchester from the man he was fighting. There was a lot of yelling and thumping, but Beckett didn't turn to look, too intrigued by the scene in front of her. Frank Walter's wife was dark-haired and athletic-looking, and in the second the lightning had lit the room, her eyes had looked like pools of black. She was smirking wickedly and didn't seem badly hurt, despite having been thrown forcefully against a wall.

And that was another thing – how could Cas throw her like that? He looked like an ordinary man (current psychotic expression notwithstanding). He wasn't enormous or heavily muscled. Ryan was right. Something about this guy was not normal.

Frank Walter's wife was on her feet, now, breathing hard. "Aww, it's so cute, your little crush on the human. Why do you care so much about that pathetic ball of stupidity and self-loathing? You should join us, Castiel. We could do with a few more falling angels."

Castiel stalked towards the woman, raising his knife.

Beckett fired in the air. "Everybody stop!" she ordered.

Surprisingly, they did, but just for a second. Then another gust of wind battered the building and Castiel continued his path towards the woman.

"Wait, Cas," a panting Sam Winchester said from where he had his man pinned to the floor. "They don't have them. Someone else got there first. Tell him what you told me." He dug the tip of his knife lightly into the man's back.

"Okay, okay. We followed them to their hideout and came back to plan." The pinned man panted out.

"Tell us where," Castiel demanded.

The man just laughed. "You know what? Kill me, Sammy. All this anger is a step in the right direction for you."

Sam began to speak, reciting the same words he'd used earlier that day when Beckett had confronted him in the Walters' apartment. The room was suddenly filled with black smoke.

When the smoke cleared, the room was still dim and there was a dead man on the floor. The smile was gone from Frank Walter's wife's face and she was swaying on her feet, threatening collapse.

Beckett aimed her gun at Castiel. "Sir, put down your weapon."

Esposito already had Sam Winchester on his knees with his hands on his head, while Ryan was crossing the room to check on the injured woman.

Castiel's sword disappeared into thin air. She actually saw it disappear. Ryan was right. He definitely wasn't human. But she didn't have time to freak out just then. Every moment that passed was a reminder that Castle was still missing.

"Tell us what's going on," she said.

XXX

"Demons?" Beckett asked disbelievingly.

"Demons," Sam Winchester confirmed.

They were in Beckett's car, lights and sirens on, following the directions of Frank Walter's wife. She couldn't remember exactly where the demon had been while in her body, but she remembered the turns and told them where to go. She sat in the front passenger seat next to Beckett, while Sam and a glowering Castiel sat in the back seat.

The sudden storm had abated, replaced by a dry wind and heavy cloud. She took it as a sign of good luck, because she couldn't think about it coming from Castiel. That was too much.

"Dean left us a message at the bottom of the fire escape. There are at least three demons and there was a live civilian with him there," Sam informed her.

Ryan and Esposito were following in their car with cans of spray paint, sacks of freshly bought rock salt and an IPod with speakers and a recorded exorcism on it.

After they rescued Castle, she was going to go home and have the biggest freak out ever.

"There," Frank Walter's wife, pointing to a brick house that stood alone in a tiny yard of bare dirt.

Beckett groaned. "Are you sure?"

"I've been a hunter for seven years. Getting possessed was a rookie mistake, but I knew what to watch for while I was possessed. That's the place. Go get the bastards."

A crowd of black SUVs took up the curb in front of the house, and the door of the house bore the telltale signs of recent battering. Several men and woman in suits and bulletproof vests waited outside.

"Crap! Feds!" Sam hit the back of the seat in frustration. "Cas?" But Cas wasn't sitting next to him anymore.

"Okay, change of plans," Sam said, as Beckett pulled her car over to the curb. "Cas gets Dean and Castle, then we go in and save the dumbass feds from the…"

Beckett didn't hear the rest because she was flinging open her door and dashing into the gap between the brick house and the one next door.

"Castle!" She cried running over to where her friend was slowly crawling out from between the houses.

"Kate," he mumbled, looking up as she knelt beside him. "I told him you were coming."

"The ambulance is on its way," she told him, sitting beside him in the dirt and letting him rest his head in her lap.

"Dean's dead," he whispered sadly.

She stroked his hair as he passed out.

XXX