A/N Sorry it's late. Please enjoy!
TARDIS
"The better question is when are we going," The Doctor said scrambling around the TARDIS console.
"I'm pretty sure he had the question right," Sam said wiping the last traces of sleep off of his face.
"Good old Castiel here said that he saw me a week ago when Mr. Crocket was kidnapped. Thing is, I haven't been there, not yet, not really."
"But Cas saw you a week ago, how could you not have been there?" Dean asked looking very confused and his hand itching to grab his gun.
"I'm a time traveller," The Doctor said, but the Winchesters just looked more confused. "People assume that time is a strict progression of cause to effect. But actually, from a non-linear, non-subjective viewpoint it's like a big ball of wibbly-wobbly... Timey-wimey stuff."
"Is that supposed to help?" Dean said looking dumbfounded. Sam looked like he understood most of what the Doctor said and he translated it to Dean.
"Cas saw the Doctor in the past but when we're seeing the Doctor he hasn't been there yet."
"Right you are Sam, when we go back I'll be able to figure out what alien we are dealing with. Then when we come back to the present we can stop them."
"Why would we come back to the present if we can stop them in the past? It would be easier," Dean said and Sam just sighed.
"Because that would create a paradox, and, believe me, those are not good." The Doctor went to go put his had on a large lever, "Any more questions?" He asked as he looked around the room. Sam was shaking his head and Dean was moving closer to one of the posts so he had something at arm's length to grab onto. "Right then. Allons-y!"
The TARDIS began to shutter and then violently jolted to one side, Dean was able to grab onto the post beside him. However, Sam wasn't so lucky and he tumbled to the ground. Cas only staggered around a bit and the Doctor took all of the wobbling and jolting of the TARDIS in stride as he was able to run around the console seemingly monitoring different systems and adjusting them. The TARDIS gave one final jolt, then made a noise signifying that they had landed.
"Are rides in this thing always this bumpy?" Sam asked getting up off of the floor.
"For the most part, yeah." The Doctor grabbed his coat from the poll Dean was hanging onto and slipped it on practically running towards the doors. Sam and Dean followed and Dean noticed Cas staying behind.
"Cas, are you coming?"
"No, the Doctor said I have to stay here so I do not create a paradox."
"Interacting with your past self can get very messy," The Doctor said, "And people can die." He added in a very low tone trying not to remember what happened with the reapers when Rose tried to save her father.
Dean wasn't satisfied with the Doctor's answer, but Cas seemed to trust the Doctor enough to stay behind in the TARDIS and Sam also trusted the Doctor. This was not an argument he was going to win so Dean kept his mouth shut.
They exited the TARDIS and Sam and Dean were surprised to be in the alleyway by the park again, but that wasn't the only thing to surprise them; it looked like the Sun had just set. And when Dean looked out to the park he didn't see his car parked on the side of the road where he had left it.
"We just time travelled," Sam said a bit dumbfounded. That box is impossible he thought.
"Yes, we did. Come on," The Doctor said turning to leave the alleyway.
They crossed the street, the Doctor didn't almost get hit by a car this time and all three of them were scanning their surroundings. Sam and Dean were looking for any signs of demonic activity and the Doctor was trying to see any signs of alien activity. They came to a stop behind the tree the Doctor had hidden behind before.
"Okay, we need a plan," Sam said.
"Right. So Mr. Crocket should be coming through the catwalk at any moment along with Castiel. Castiel only remembered seeing me not you two, so do not let him see you."
"So you just want us to stay here?" Dean asked.
"Yeah, pretty much, unless something goes wrong."
"What can go wrong?" Sam asked.
"I have to get pretty close to our alien friends to get a strong enough reading so I can figure out what species they are."
"Awesome," Dean said sarcastically, "Do we know how many alien-demon things are going to be here?"
"No, Castiel never mentioned a number."
"Awesome," Dean muttered again.
"So you're just going to run into the catwalk, with no weapons, and hope you can figure out what the aliens are," Sam said.
"Yep."
"That's not a very good plan."
"Have you got a better one?"
To this the Winchesters were silent; they were out of their element chasing down aliens, not to mention time travel. They had done it before but never in such a small jump. Dean was about to say something when the Doctor spoke.
"Here he comes."
Sam and Dean exchanged a look. Then they heard Mr. Crocket whistling from the other end of the catwalk. Then they saw three people enter the catwalk from their vantage point.
"That's my cue," The Doctor said as he slipped into the catwalk behind the three people that had just walked in.
"What are you guys doing?" Mr. Crocket called. The three people had stopped in front of him and looked like they were about to charge him when someone came up from behind and hit him over the head.
Castiel had been watching this all unfold from behind Mr. Crocket, there were six people in the catwalk now, too many for Cas to take out without help. Then he heard a weird beeping noise, apparently so did everyone else in the catwalk as they all turned around.
"Blimey." The Doctor muttered as he read the results off of his sonic screwdriver. Then he looked up and saw all of the eyes in the catwalk on him, "Hello. Is this how you Americans spend your time? That old chap looks like he needs a hospital! I'm a doctor, let me take a look at him."
"It's none of your business!" One of the people snarled and they started moving towards the Doctor. Castiel was now able to get a better view of this mysterious man, there was something off about him but Castiel couldn't figure out what.
Meanwhile, Sam and Dean were behind the tree near the mouth of the catwalk, Dean wanting to run into the catwalk and kill some evil sons of bitches while Sam was trying to figure out what exactly the Doctor was doing.
"Oi! No need to be rude! I'll just be off then, just make sure he's okay." With that final statement, the Doctor turned on his heel and walked out of the catwalk. He wanted to save Mr. Crocket, but that would mess with the time stream too much and cause problems he didn't particularly want to solve. Once the Doctor was out of the catwalk he saw Dean holding his gun; it looked like he was about to storm into the catwalk and start shooting things.
"I figured out what they are. Let's head back to the TARDIS."
"But we can save Mr. Crocket now," Dean complained tightening his grip on his gun.
"That would have catastrophic effects on our future," That clearly didn't please Dean, "I want to save him to but I can't. We have to go." The Doctor turned around and headed towards the TARDIS keeping to the shadows.
"Come on Dean. I don't want to get stuck back in time." With that, Sam turned to follow the Doctor and Dean reluctantly followed Sam.
Castiel had been thinking about the British guy, how does he fit into the puzzle? He didn't give a name, all Cas knew was he was a British doctor. And what was that strange device he was using? All Cas knew for sure is that he had his work cut out for him.
TARDIS
Castiel had been waiting for what seemed like forever in the TARDIS then the Doctor ran in and Sam and Dean walked in after him.
"Are you going to tell us what alien we're dealing with?" Dean asked, still pissed off at the Doctor.
"They're the Laplji," The Doctor said pulling up a grate on the floor.
"The what?" Dean asked.
"The Laplji, from the planet Tupljak. They are similar to demons in the sense that they need to possess a living organism to interact with things. Ah-ha!" The Doctor exclaimed as he pulled a mechanical-looking jar out of a large box that was underneath the TARDIS console.
"What's that?" Sam asked.
"Good question. It's a contraption used to hold the Laplji if a compatible organism cannot be found. They can't survive too long when they're not in one."
"Then what's with those weird control thing that they carry?" Sam asked.
"The Laplji didn't always need to possess organisms but after an unknown incident, they made a technology to aid them in the process. But I am pretty sure they only use the technology in emergencies, I'm not sure why they're using it now."
"So it's not a weapon," Dean asked.
"No! What's with you and weapons," The Doctor said, "Don't answer that."
"Let's head back to our time and figure out our next steps," Cas said.
"Wonderful idea! Allons-y!"