Booth was in the process of securing an elementary school teacher to the monkey bars with his belt when the sprinklers went off. She was one of the women from Mrs Sparrow's afternoon tea circle. They'd questioned her two days ago. She'd been excessively smiley and given Booth the creeps. His opinion wasn't improving. She wasn't a demon, but she had some nasty fingernails and was going for his eyes. He'd run out of cuffs ten minutes ago and was really, really ready to wrap this whole thing up. It didn't seem like that was going to happen anytime soon though. He'd caught a few glimpses of the Winchesters since the explosion and they seemed vastly outnumbered.

The sprinkler came on directly beneath his feet just as he finished buckling the teacher firmly to a bar. The spray was fine and misty, catching the red light of the fire as it hung in the air. He was torn between being glad for the coolness and annoyed that the ground would be slippery. He weaved through the crowd to Bones, who was holding her own against a man in a grey suit, but looked tired.

The clamour started slowly, and it wasn't until Booth had reached Bones and helped her cuff her guy that he realised what was happening. The fine spray was hitting the possessed, stopping them in their tracks. They would pull up short, stand still for a moment, before jerking about oddly like they couldn't control their heads. Some, the ones with the stronger demons in them, he supposed, would recover, stumbling off towards the edges of the fields or the podium. Others would jerk more violently, clasping their faces in pain. Some steamed and cried out. The black smoke would fly out of their mouths right after that, disappearing into the night. Booth didn't know where it went, but he had more immediate concerns.

The people who had been possessed almost all collapsed. Booth and Bones rushed to help. It was too late for some – others were just exhausted and confused. Booth was helping one – apparently a receptionist at the school – to the driveway, where the fire service and ambulances were just pulling in, when he noticed something. The kids, all the ones who hadn't managed to get out in the big rush, were falling to their knees, vomiting up black goo. Lots of them were crying and alone, separated from their parents in the rush. He delivered the receptionist to a paramedic, and ran back to gather some kids. He nodded to a guy he knew from the bureau, and tried not to think about explaining all this.

He was kneeling beside a crying kid, who couldn't be more than five, trying to reassure him, when suddenly he wasn't. Without warning, there was a hand on his head, and a nasty sucking feeling around his body, and suddenly he was sprawled on damp grass in the middle of a brawl. A brawl the Winchesters were losing, despite the twitching of the demons as the holy water affected them.

"Reinforcements. Excellent," said Dean hoarsely. He had his knee in the back of a large man who appeared to be having a mild seizure. There was black goo dribbling down Dean's chin and over the head of the demon he held down. He wiped his mouth with his hand and rubbed it dry on the demon's shirt. "Ugh. Brainwash goop." Then he punched the demon hard in the head and it went limp. Dean got up. "Exorcise it," he ordered Booth.

Booth yelled the exorcism as forcefully as he could. Smoke streamed out of the man, but there was no time to celebrate, because a demon was already on him, and it was all he could do to fight it off. His muscles burned. His fists ached. His knuckles bled. He was too old for this.

They were all there. This was the big one, the final battle. If they didn't win this it was the end of the world. And so Booth fought through the pain and kept going. And anyway, it looked like Hodgins had a broken arm. If he gave up before a guy with a broken arm, that was just embarrassing. Especially if that guy was Hodgins.

Hodgins and Angela were teaming up on the demon with the smallest host. They were yelling "Christo," over and over and Angela had somehow tied its hands behind its back and they were dragging it out of the melee, just to get out of the way.

Bones had her holy-water pistol out again, and was spraying it straight down the throat of the guy she had on the ground. He writhed and screamed and the demon flew out of the host's mouth. She dragged the body out of the way.

Numbers were falling. The battle was swinging back in their favour. Suddenly, it wasn't ten on three, as it had been when Booth had seen them earlier. It was six on six, and Booth nearly had his.

Castiel shoved his guy against the podium, snarling as he placed a hand to the host's forehead and killed the demon in a flash of white. He looked ferociously smug and turned to see who was next in line.

A woman with black eyes had Sweets pinned to an oak tree; a hand creeping up around the psychologist's throat, but Sweets was fighting valiantly. Booth saw red seeping through her jeans where Sweets had dragged his short blade across he leg. She faltered.

But Dean had two on him, and was already nursing bruised ribs from the day before. They weren't politely attacking one at a time. One was holding him still and the other was punching him, hard and methodical, and taunting him quietly. Booth couldn't hear what was being said, but from the look on Dean's face it was cruel. Dean's expression was hard and angry, and he was struggling and kicking the demon that held him, and he looked murderous.

Booth wanted to help, but the demon he was fighting chose that moment to punch him hard in the face, interrupting the flow of his exorcism. By the time the smoke disappeared into the ground, Sam was at his brother's side, blood dripping from a strangely engraved knife he grasped tightly in his right hand. He was breathing hard and his face was stony, his eyes dark and flashing. His hair had blown across his face in the fight and he hadn't bothered to put it back.

Everyone stopped and looked at Sam. Sam pulled the second demon off Dean and stabbed it hard, straight in the heart. No hesitation. No empathy. No compassion. Booth fought the urge to run. Grab Bones and run. Hide in an underground bunker somewhere and never come out.

The sprinklers spluttered and died. The water had been diverted to put out the fire.

One of the two remaining demons started to laugh. "You liked that didn't you, Sam? It's such a rush, feeling the knife pierce bone... and that blood... can you hear it calling, Sammy?"

Sam swallowed. His eyes were narrow and angry. He stepped towards her, the knife gripped tight in his bloodstained hand.

"Wait," Dean grabbed his brother's arm, "This is a trick. This is what they want you to do." He coughed and spat blood on the ground, but he stepped between his brother and the demon and stood squarely in Sam's way. "Remember what happened with Lilith? Sometimes killing them isn't worth the satisfaction."

"Feel that anger, Sammy. Listen to the blood singing in your veins. Smell that blood. You know you want a drink."

"Dean, get out of my way."

"No."

"I really want to kill him, Dean."

"Don't do it Sam. I mean I'm all for killing demons, but the more you do it, the better it feels, and that what they want. They're using that feeling to get you to go dark side, and we all know you're not really like that. Right?"

The question was put out to the squints in general, and for a moment it just hung there. The words caught in Booth's throat. He didn't know what Sam was really like. He'd only known him a few days, and in those few days Sam had tried to kill him and several of his friends.

Castiel stepped up, though. "We don't have time for this, Sam. Leave him alive."

"Well, that was helpful," Dean muttered.

But then Sweets pushed the female demon off him and walked over to stand beside Dean. "Sam," he said, "I find you very interesting. You have issues, certainly, but nothing that cannot be coped with. I can help you – even if you just need to talk. Any time. And I know you have lots of awesome stories and ideas. I've only known you a couple of days and I've never met anyone quite like you. We all want to get to know you, and it would be a huge waste if you gave up on us. Please don't do this."

Booth's brain jumped back into gear, and he stood beside Dean and Cas and Sweets. Bones followed, and finally Angela and Hodgins, holding hands.

Sam seemed to shrink, and his eyes brightened. He wiped the knife carefully on his shirt and handed it to his brother. He asked nervously: "Do you think I could maybe hang around the lab for a while? I've always been interested in forensics..."

Dean called him a geek in an affectionate older brotherly sort of way, and Booth could just see Angela squealing with delight on the inside at the sweetness of the moment. Although both brothers would obviously deny any sort of 'moment' until their deathbeds. Sweets smiled quietly to himself in that smug way he got when Booth or Bones admitted something private in one of their sessions.

"Isn't this sweet?" The demon that had been goading Sam interrupted.

Dean turned and drove him to the ground in a display of extreme violence that must have been hell on his ribs, and held him there as Cas did the white light thing.

Cas turned to the last demon, but smoke was already spewing from the vessel. The demon had done the smart thing and abandoned ship while it still could.

And then it was all over but the cleanup.