The Doctor walked alongside Sam in silence for a long time. The silence didn't seem to bother Sam, who was intent on what he was doing and more or less seemed to take the Doctor's presence as a side detail instead of trying to engage with him. And that was weird enough to be concerning, but the Doctor was more than happy to take advantage of the silence to put his mind to the problem in front of him.

Sam had always had a distinct psychic impression on the space around him, ever since he was a kid. But this wasn't that. This was an empty space. It wasn't even a drain. It was… nothing.

"So, what's the hunt this time?" the Doctor asked after confirming, again, that he really had felt nothing from Sam. He wasn't sure if this was Sam. He didn't see Dean anywhere nearby. "Where's Dean?"

"Looking for you," Sam said simply. The Doctor had never heard him sound so much like he didn't care. Even the times that the Doctor had seen the worst of them, their flaw was that they wanted to protect each other, not … this.

"Well, let's find him before he gets lost," the Doctor said, but Sam shook his head.

"He can follow the same trail I'm following," he said. "And we can't let this thing get away."

The Doctor frowned and looked behind them. "And what is this thing?"

"Rugaru," Sam said, the word spat like a curse. But even if his features twisted in distaste, it was too much, like he was putting on a show to act like he was disgusted with the thing he was hunting. He was mimicking emotion, but the Doctor could feel nothing coming from him.

The Doctor narrowed his eyes and then came to a stop, though Sam didn't stop with him. "And what are you?" he asked, crossing his arms.

Finally, Sam turned back to look at the Doctor, raking his gaze over him, looking for clues to how he should react. "What are you talking about?" he said at last.

"Just what I said. You've taken Sam Winchester's form, but whatever you are, you're not him," the Doctor said. "You're empty. Whatever you are, you have the form but not the heart."

Sam stood up straighter and narrowed his eyes. "I am Sam," he said.

The Doctor tipped his head nearly sideways and uncrossed his arms, walking a slow circle as Sam stood still, watching him the way a predator would, coiled and ready to strike if need be. "I think you believe that. But even if you are Sam, what's happened to you?"

"Really?" Sam gestured with both hands. "Come on, now, Doctor. You know the story of my family. Don't tell me you don't. The life of a time traveler, remember? You knew long before we did what was coming for us, and you know how Dean and I stopped Lucifer and Michael." He paused. "How I stopped them," he said, and the Doctor suspected that his smile, his pride was only there because Dean wasn't. There was an echo of emotion there, but it was warped.

The Doctor stopped circling him. "Whatever happened, let me fix it."

Sam narrowed his eyes and then turned around to go back to what he was doing, moving with his whole body. "I don't think it needs fixing, Doctor. Besides, we have more important things to do."

"More important than—"

"Yes."

The Doctor narrowed his eyes but continued to stay back, watching Sam go to work. The boys were always concentrated when they were on a hunt, but again, this was different.

But Sam picked up speed, and the Doctor had to jog to keep up with him. Whatever was going on, he wasn't going to leave Sam alone on a hunt. He kept looking to see if he could spot Dean—he had so many questions—but how would they even find Sam if they got separated? Would he even care to look for them?

And why was he even sticking around Dean if he had lost the part of himself that cared? By the Doctor's recollection, this would have been about the time that Dean had tried to quit the life. He'd had a cute little house, neighbors not withstanding.

Was this why he'd left all that behind?

Then again, he totally understood the impulse. He couldn't resist the mystery that was Sam Winchester now. Just like he couldn't resist trying to figure out what was going on with Clara.

And that was a whole other train of thought. He still felt terrible about what had happened. And he wanted to figure out that mystery. He couldn't believe that Dean Winchester's final fate was to be a demon. There had to be a way to fix that. He just hadn't figured it out yet.

At the very least, he could start researching in his spare time. Or looking into a way to minimize what was happening. Maybe he could figure out whether or not the curse lasted past the death of the Earth—but then, time displacement could do funny things, and he didn't fully understand curses anyway.

And if Dean was trapped with impulses of violence, with the worst side of him, maybe there was someplace he could go so he wouldn't hurt anyone, so that if he ever did get away from that curse, he wouldn't look back on everything that happened while he had the Mark with the level of guilt the Winchesters were known for.

He wanted to do it. He wanted to save both of him. And since the Doctor had seen the Winchesters around later in their timeline and hadn't sensed this emptiness in Sam, maybe this was his shot to learn more about curses, to figure out how to free them. Maybe this could help him with the Mark. Maybe it was similar.

Or maybe he was just hoping for answers because he was an incurable optimist.

He sighed to himself and then stopped when he saw Sam look back and glare at him for making too much noise. He had been good at stealth in previous bodies, but this one … not so much. Especially not when he was this distracted.

The Doctor held up both hands in a conciliatory gesture, and Sam rolled his eyes but went back to what he was doing.

And as for the Doctor, he followed closely, his mind turning over the possibilities. But when he heard a piercing scream split the air, he had to table that particular train of thought. At least for the time being.

There was a more immediate need.