It's only when he has Dean tied down in the backseat of the Impala that Sam realizes he hasn't thought about how he's going to cure him. He knows the ritual, of course—injections of sanctified human blood every hour, and a special exorcism—but he hasn't thought about where he's going to get the materials. Because, he reflects now, he isn't so sure his own blood will work.

From the moment he'd discovered his brother was a demon, he'd been racing to follow every possible lead before Dean could go to ground completely. There hadn't been time to even attempt to tackle the logistics of stealing from a blood bank. Sam might not have balked at the idea of kidnapping some innocent person off the street—some ignorant, innocent, pure person—after all, he hadn't balked at much of anything during the hunt for Dean; but there wasn't even enough time for that. And now that he has Dean, the only thought in Sam's head is to get him secured in the bunker before he can vanish again. Sam's blood, tainted and not-quite-human though it may be, will simply have to do for the ritual.

It worked on Crowley, Sam tells himself firmly.

That was during the trials, though, his mind chimes back at him. Closest you've ever been to being pure.

Pure and dead, Sam snaps back. Sometimes Sam still resents Dean for tricking him back into life, but right now he's grateful. Can't help Dean if you're dead. Of course, that's assuming he can help Dean. But after all, they're family, and isn't that what family's for? To bleed for each other? Sam figures that should count for something.

*S*P*N*

Dean doesn't struggle when Sam brings him down to the dungeon and locks the spelled shackles around his wrists, ankles, and neck, chaining him to the chair in the center of the floor. He just sighs and rolls his eyes in exasperation.

"You really think these are gonna hold me, Sammy?" he asks, and he sounds so like and at the same time so not like the old Dean that Sam shudders. "Please. You know, I got a hell of a lot more running through me than just demon juice."

"Mark of Cain, got it," Sam says, in the most neutral tone he can muster.

"That's right. Matter of time before I'm outta here."

Sam ignores him and lines up eight syringes on the side table. Taking hold of the first one, he carefully pricks the needle into the crook of his own arm and draws the plunger upward. Dean starts to laugh, and for the first time in his life, Sam is chilled by the sound, rather than gladdened.

"You're using your own blood?" Dean sniggers. "Sam, didn't they teach you any logic at that fancy college? No way is adding more demon blood gonna work to make me human."

"You said time was of the essence, didn't you?" Sam says as he removes the needle from his arm. "I need blood for the ritual. This was the first available supply."

With that, he stabs the needle into Dean's neck.

*S*P*N*

Over the next few weeks, once his triumph at the success of the ritual has faded somewhat, Sam watches. He watches Dean cleaning up the Impala, lovingly polishing her back to a pristine gleam; he watches as Dean settles, a little awkwardly, back into the hunting routine. There's a tense moment with a shifter in Connecticut, and Sam realizes the Mark of Cain is still a problem they need to work on, and he watches. Dean goes right back to normal afterwards, puttering around the bunker in his "dead guy" bathrobe, making hamburgers in the kitchen, humming Metallica as he searches through the library.

The cure worked, Sam tells himself. It must not have mattered that his blood wasn't perfect; what must have mattered was that his blood is Dean's blood; he must have been right when he figured that should count for something. Still, when Castiel calls on them to help track down Claire Novak, and Dean acts his usual obnoxious self, making sardonic remarks and devouring any kind of fast food he can get his hands on—still, Sam watches.

He doesn't know what he's watching for, exactly, until he hears screams issuing from the house where Salinger and his goons had been holding Claire. Sam looks around, realizing suddenly that Dean hasn't followed him out to the car; he's still inside. A vague terror welling up inside him, Sam runs back to the house, finds Dean kneeling at the center of a circle of corpses, and the first thing he does is try to look in his brother's eyes. That's when he realizes he's been watching, waiting, for them to flip back to demon-black.

Dean avoids his gaze.

"Tell me you had to do this," Sam says. He's crouched in front of Dean, a hand on the back of his head, trying to force him to look up.

"I didn't—" Dean says, and pauses, as though wishing he could stop there. Wishing the denial were true. "I didn't mean to."

"No," Sam growls, letting anger mask his fear. "Tell me it was them or you!" Dean finally looks up at him, and his eyes are green, green, not black, but he doesn't answer, and he looks away again quickly.

*S*P*N*

After that, Sam starts to think he was stupid to suppose that something like family could overcome the flaw in his blood, that any confession could absolve away the demonic taint that's on him. He wonders if he failed his brother, yet again, by not using someone else's blood, anyone else's blood. He should have made plans to get some, he should have taken the risk of leaving Dean chained in the dungeon, but he had let his emotions cloud his judgment, just as his father had always warned him against doing, and as had always been his wont. And now it appears that the ritual hasn't worked as well as he'd thought. Perhaps it had only been good enough for a temporary fix; perhaps Sam had doomed Dean to become again the twisted, empty thing he had been.

They're both sitting in the library at the moment, scouring the news for potential cases. Dean's in front of the laptop, frowning in concentration, while Sam pores over the newspaper, doodling absently in the margins with his pencil. Dean looks over at him, and Sam braces himself against a thrill of dread before looking back. Dean's eyes meet his—green, not black—and he lets out a small breath of relief.

"Listen to this one," says Dean. If he notices Sam's tension, he doesn't let on. "A death row inmate in Livingston, West Virginia vanished from his cell. Got out without triggering any alarms, no security breach, no guard misconduct. What do you think?"

"Yeah, let's check it out," Sam agrees, trying to keep his voice light. Dean flips the laptop closed and gets to his feet, heading out of the library towards the bedrooms to grab his duffel. As he passes over the devil's trap laid into the floor by the doorway, Sam grips his pencil so hard it almost snaps in half.

*S*P*N*

Sam's hardly surprised when the case turns out to be bigger than expected, because if there's one thing he's learned it's that he and Dean can never catch an easy case. He's also not very surprised when Dean insists on facing Cain alone. He continues to offer his help, because that's what family's for, but isn't surprised when Dean continues to refuse. After all, lately it seems like family doesn't count for much of anything.

*S*P*N*

Sam doesn't know what Cain said to Dean, but he knows it rattled him. He wishes more than ever he could see some way to help, but he can't, no matter how long he lies awake thinking about it. He's already given his own blood to save Dean, and it's becoming clearer and clearer that it didn't work. With a sigh, Sam rolls out of bed and makes his way down to the kitchen. Alcohol won't help him think, but it might help him get to sleep, at least.

It turns out he's not the only one having trouble sleeping; Dean's already there, sprawled in a chair, a bottle of whiskey and a tumblr on the table before him. He looks around at Sam as he enters, and Sam automatically checks his eyes. Green, not black. For now.

"You look like you could use a drink," says Dean, and pushes the bottle of whiskey across the table as Sam drops into the chair opposite him. Sam doesn't deny it—it's what he came down here for, after all—and takes a swig directly from the bottle.

They sit in silence for a while. Sam wonders how long it will take Dean to ask him what's wrong, and if, when he does, Sam will be brave enough (and drunk enough) to tell him.

"You don't have to worry, Sammy," Dean says, and Sam wonders for a moment if he'd been speaking his thoughts aloud. "I'm not gonna be that thing again." He leans over to punch Sam's shoulder lightly. "I've got you to keep me human, don't I?"

Sam shrugs away, his gut clenching. "I don't know if I'm the best person for that," he mumbles.

"You're my family," Dean says. "That's gotta count for something, right?"

Sam looks up, surprised, to meet his brother's eyes—clear green, and smiling. And suddenly he thinks that maybe his blood worked better than anyone else's could have.