Tennis balls were mankind's greatest invention. Castiel could not believe he had never noticed how good and beautiful and perfect and wonderful they were before now.

Including stopping to eat, sleep and get gas, it was an almost two day drive from Canon City to the spot on Redwood Highway that Castiel pointed out. Dean, impatient in his usual way, wanted them all to take the Impala together and just drive straight through, switching drivers as needed, but Sam and Mary talked him out of it by reminding him that there was a very good chance that visiting the "scene of the crime" wouldn't really help much in this case, and it didn't make sense to burn themselves out chasing what was probably a wild goose.

So instead they formed a convoy, with Dean driving the Impala, Sam shotgun, Castiel in the back and Mary in her car behind them. Nobody for a moment entertained a different arrangement, even though they certainly could have. Habit made them do what was comfortable for them.

After spending the night in a motel, Dean and Castiel were up earlier than the others. Dean had secured a tennis ball from somewhere (Castiel found he did not care where) and was idly tossing it into the spread of grass next to the motel parking lot, his hip leaned against the Impala.

Castiel had not initially understood the purpose of the tennis ball. But then Dean had thrown it the first time, and all confusion fled instantly, as did any thought save the one of running the ball down and capturing it, forcing submission from it and then… well it took him a moment to figure out what to do after that. He brought it back to Dean, looking for an explanation. Dean ordered him to release the ball. Against every joyous instinct that had been touched by having the ball, he complied.

Wonder of wonders, Dean threw it again, and Castiel felt a massive thrill shake him from nose to tail as he whipped around and ran after it all over again. It was as though his entire purpose, his entire reason for being, the summation of all that was good and beautiful in the world, was tied up in chasing and seizing that ball. And, if he gave it back, then Dean would throw it again. It was the pursuit and acquisition, rather than the actual possession, of the ball that was truly exhilirating, so after that first time Castiel did not spare a moment of indecision or hesitation in sprinting back and depositing the ball in Dean's hand to be thrown again.

Castiel had learned to fetch, and he loved it with everything he had, more than he had ever loved anything in the world. Except his adopted Winchester family, of course.

On one of his return trips with the ball, Castiel realized that Mary had joined her son. He broke out of his run and paused uncertainly. But a moment's watching told him that this was not a private parent-child moment, it was just a casual Hunters at dawn socializing moment. Relieved that he would not be denied the opportunity to chase the ball again, he brought it eagerly to Dean.

"You shouldn't do that to him," Mary said, her expression soft with concern, "He's an Angel."

Dean offered her a half-smile, looking down at the excited collie face, the eyes that watched the ball so intently, the ears flung forward in anticipation and inquired, "Have you ever been part dog?"

He bounced the ball in his hand, and Castiel started to back away, expecting to visually follow its flight path, as he raced after it over the frosty grass. But instead the ball came back down into Dean's hand. Frustrating. Trying Castiel's patience. He resisted the urge to bark.

"Have you?" Mary asked in some surprise.

Dean's smile became a broad grin that was more eloquent than a verbal reply. And then threw the ball!

Castiel forgot about Mary, forgot about the human words, forgot about the fact they were talking about him, forgot who, why, when and where he was, his mind filling with the urgency of pursuing the ball and obtaining control of it before it hit the ground, which was a greater thrill than simply snatching it as it bounced and rolled, and a thousand times more satisfying than waiting for it to stop entirely.

On his return trip, he found they were still talking, and about much the same subject. Why couldn't they talk about something meaningful and pure… like tennis balls?

"Look, all I'm sayin' is that Cass has had to put up with all the crap that goes with being a dog," Dean was explaining, as Castiel passed him the ball and then stepped back to wait, "He's been tied up, locked up, yelled at and half-starved," it was clear he'd nearly included being kicked on the list, but couldn't quite make himself say it out loud, "The least I can do is let him experience one of the good things about bein' a dog," he waved the hand with the ball, and Castiel followed it with his nose and eyes, "It's like..." it took Dean a moment to think what it was like, "Like drinkin' a beer or watching TV," he threw the ball as he concluded, "It's cathartic."

It was Castiel's considered opinion that Dean had no idea what he was talking about. Beer and television had nothing on fetching a ball. Castiel had fallen in love with television gradually, at first it had mystified him and later he had found it was a welcome distraction from weakness, exhaustion, pain… and being possessed by Lucifer. But television and tennis balls… there was no comparison between them. None.

Of course, Castiel heard what Dean wasn't telling his mother. About how when Castiel's Grace had been stolen, he had seen the hardest parts of human life. Castiel had known hunger and thirst and being hopelessly lost and practically helpless and awash with needs and cravings and desires that had never been his before, that he didn't understand or know how to cope with. He had felt the pain that came with social awkwardness, and romantic rejection, and the sufferings of a mortal body when it was dehydrated, bruised, starved, strained, tortured and killed. And it was Dean's feeling that he had abandoned Castiel during that time of greatest need, had failed to help him learn about and cope with his new, extremely fragile, existence.

Whether or not that was true, Dean had done it to protect Sam, which was a good and noble reason. Though it had all turned out rather messy and nothing had been as it seemed, Castiel would willingly endure that hurt and bewilderment and fear again for Sam, Dean or Mary at a moment's notice. But still, Dean felt guilty about it. Castiel could hear it in the man's voice. and he knew that the real reason Dean had promised twice over not to abandon him now was because that memory of when Castiel was rendered for all intents and purposes human remained fresh in Dean's mind, and the guilt of it was a pain he would never let go. He thought Castiel should hold it against him, or at the very least be afraid that it might happen all over again. But Castiel didn't, and he wasn't.

Just let me keep chasing this ball forever. That's all I want.

But when he brought the ball back, he saw that Sam had also surfaced, yawning at the door of the motel room. He knew Dean would not throw the ball again. Instead, the group would have breakfast together, and then they would hit the road. The game of fetch was ended.

Swallowing his disappointment, knowing that he should be grateful because the Winchesters were out to help him, not realizing the looming threat to themselves that Castiel still had found no way to warn them of, Castiel returned the ball to Dean's hand for the last time.

Unthinkingly, Dean ruffled the fur between his ears. It felt surprisingly good, and the disappointment at not playing fetch anymore evaporated as he began to think about and look forward to food instead. A mind capable of focusing on but one task at a time was one of the small mercies of being a dog.

Over the course of breakfast outside a small cafe whose name Castiel hadn't cared to read, the Winchesters tried to pretend everything was normal. But they were terrible at small talk, and inevitably they started going over Castiel's case, detail by detail, what little they knew of it. They even asked him questions he'd already answered. Castiel was patient in his responses, well-used to their way of trying to work through unsolvable problems. He also knew they had a way of surprising him, of finding the answer when there hadn't even seemed to be one.

He lay on the ground beside Dean's chair, pretending to be an ordinary dog for the sake of the nearby civilians, some of whom whispered to one another about leash laws. But then he heard it. A rumbling, mechanical growl. Castiel sprang to his feet, looking around wildly until he saw the motorcycle backing out of its parking space. His body went rigid as he stared at it.

"Cass?" Dean's voice came from a tremendous distance, and seemed… unimportant.

The motorcycle turned and started out of the lot. Castiel felt his body vibrating as an irresistible excitement, an instinctive demand, took control of him. He didn't just want to chase that motorcycle, he had to. Every inch of him quivered with the absolute need, his mind flooded and drowned in that singular, senseless imperative. As the motorcycle rumbled onto the road with an ugly sputtering noise, Castiel could take no more. With a savage bark, he launched himself. He felt, briefly, a hand touch his back, but the fingers slipped uselessly through his fur, and he completed his leap unhindered, covering fifteen feet in a bound and hitting the pavement running.

"Cass, no!" was that Dean? He wasn't sure, and couldn't stop to check without losing sight of the motorcycle. And he must not lose sight of the motorcycle.

He raced across the empty parking spaces, gaining speed with every stride, took the downward slope of pavement at a gallop and swung onto the street at a dead run, barking hysterically and wholly unaware of it, forgetting his recently developed fear of cars, forgetting everything in the moment except for the imperative that increased with each beat of his canine heart like an urgent command: Chase the motorcycle. Run it down.

Castiel was in that moment a complete idiot, possessed of the weakness that had plagued the herding breeds since the invention of the motorized vehicle, particularly when they were deprived of actual livestock to chase and control. To such dogs is given the weakness of seeing cars, trucks, motorcycles and bicycles and other fast-moving objects as surrogate sheep or cattle, and the urge of their kind is to pursue and bring to heel such beasts, regardless of the effort it takes or dangers involved. It is their all-consuming passion, the drive above all other drives which motivates them beyond training, beyond reason; the instinct that surfaces when they are mere months of age or does not, and if it does not than the shepherd or cowboy knows that the dog will never be good for herding, because there is no way to instill in a dog the instinct for herding, and there is no substitute for that innate desire and talent that forms the basis for the hard work and training that will eventually make a dog of herding breed into a dog of herding character. But when not shaped, not molded, not directed, the instinct finds its own outlet. And so it had with Castiel, who had, without his knowledge or consent, become all at once an incorrigible motorcycle chaser.

"Cass!"

But he was deaf to the human cry, his ears were tuned only to the sound of the motorcycle, which he pursued to the exclusion of all else until it finally managed to outdistance him, disappearing first from sight, then from hearing. But he could still smell it, and followed it for several minutes more, before his clouded mind cleared, and he stumbled to a confused stop at the side of the road. And there he remained in a paralysis of bafflement until the Winchesters caught up with him.

"Cass, what the hell?!" Dean demanded, rolling down the window of the Impala to shout at him.

Dean knew, somehow, that Castiel's behavior had not contained thought or purpose. He had divined the difference between a lunatic action and an unexplained one, and he knew that this had been the former. Castiel looked up at Dean helplessly, his tail tucking itself between his legs without his wanting it to. He was sorry, and he had no excuse, nor any means of explaining what had come over him, in part because he simply didn't know. It was something that had never been a part of his nature, and it terrified him because it was just one of so many things that triggered bizarre behavior in him that he couldn't control, that snatched reason from his head and made him frantically mindless.

"Okay," Dean said, calming down now that his panic over potentially losing Castiel altogether was subsiding, and seeing that Castiel hadn't meant to cause trouble but was helpless to avoid doing so, "Okay, I didn't want to do this, but we're gonna have to get you a leash, Cass."

"Dean!" Sam objected from the shotgun seat.

"I'm sorry, but it's not safe. Not for him and not for us," Dean replied fiercely, "Especially if he goes… you know… full dog," Dean bit off each word because they hurt him to say, but didn't stop, "He'll get himself killed. He can't help it. But we can help him."

"By keeping him prisoner?" Sam asked, offended by the idea.

"Hey, it ain't like we've never had to make this choice before," Dean reminded him.

It was true. Countless times, Sam had locked up Dean or Dean had locked up Sam because one or the other had lost some or all of his bag of marbles or his soul or humanity, and they had done it for their brother's own good. They had even locked Castiel up at least once to protect him, themselves and others. Castiel remembered. He understood. He would accept a collar and leash, for his own protection, as well as Sam and Dean's. There was no telling what he might do next time he saw a motorcycle, and Heaven forbid anyone besides Dean showed him a tennis ball.

The rest of the journey to where Castiel had been changed was largely uneventful, marked by long hours of travel, eating, sleeping and little else. Castiel did a great deal of sleeping now, it seemed his new form spent two thirds of its time doing that, and the other third being very excitable, trying to chase anything that moved and thinking about food and how to get more of that. It was getting harder all the time to overcome the impulses that drove his body to act as it did, harder to think, to keep a clear head, to do what it was that needed doing, to remember what it was he was trying to accomplish.

Somehow, sooner rather than later, he had to find a way to warn the Winchesters about what was out there. The time when he would be unable to even try was coming, faster all the time.