It was a Roleplay at first so excuse the format. Part 2 is written normally since the RP died and I wanted to continue it. This is a crossover between Doctor Who, Supernatural and Gravity Falls

Sigma: I think your song should end.
Carmen: It is returning through the dark.
Sigma: Every song must end.
Carmen: You will know when you hear him knock four times.
Sigma: This is what we've seen Doctor, the darkness heralds only one thing...The End of Time itself.
The Doctor snapped awake inside the TARDIS. He looked around as he wiped the sweat from his head. The dream he had about Carmen and Ood Sigma's tied messages worried him. He jumped up from his chair and starting setting his next destination...Earth, more specifically Oregon. Sam: Where are we going this time?
Castiel: Oregon, there's something important there...I can feel it.
Dean: Good feeling or bad feeling?
Castiel: I don't know...it's just a feeling...Like we're supposed to be there.
Sam: Better not waste any time then.
Dean: How long is it gonna take to get there?
Sam: Not long. Two hours at most.
Dean: Well then I'm gonna get some sleep.
Sam: It's 7AM Dean: Exactly, good morning.
Dean closed his eyes and after a minute went to sleep.
Dipper was reading through his book when he stumbled upon a set of four pages that caught his eye. These pages talked about a lord of time who'd appear in his magical blue box when the people of Earth were in need of help.
Mabel: Dipper!
Her sudden appearance startled Dipper causing him to fall out of his chair. She picked up his book and read the section he was reading.
Mabel: Ooh Timelords, magic boxes and aliens!
Dipper: Mabel don't sneak up on me like that ever again.
Mabel: I didn't sneak up on you I walked up to you. Maybe if you weren't reading this sci-fi stuff you'd know that.
Mabel said as she handed him his book back Dipper: I can't help it if I get sucked into good reading.
The twins walked outside and into the forest.
Mabel: You don't think he's real do you?
Dipper: What? No of course not. It's just a story written up. I mean aliens? Come on there's no such thing as aliens.
Mabel: Mh-hmm.
A few hours passed by as Dipper and Mabel wandered the woods.
Mabel: We're lost aren't we.
Dipper: No, no I know exactly where we are.
Mabel: Really where? Dipper: We're...uh...okay yeah we're lost.
Sam, Dean and Castiel walked through the woods as well close to Dipper and Mabel.
Sam: Cas are you sure this is the place? It looks like a peaceful scene to me.
Castiel: We're getting close.
Sam: Close to what?
Dean: Whatever's gonna try to kill us next probably.
Mabel: Well why didn't you just say so?
Dipper: I thought I knew where we were.
Mabel: Maybe if we get up on one of the tree's we can see where we are!
Sam: Wait...listen...
Dipper: That's a terrible idea for a lot of different reasons.
Mabel: I don't see you coming up with any idea's.
Dipper: I'm thinking, i'm thinking!
Sam: Voices over there.
Dean: Let's check it out, it could be what's got Cas' attention.
Sam, Dean and Castiel walked towards the sound of Dipper and Mabel's voices until they came upon them.
Sam: Just a bunch of kids...
Mabel: Hey do you guys know a way out of these woods?
Dean: Cas did you bring us here to save a couple of kids lost in the woods?
Castiel: No it's something bigger, more important than that.
Dipper: Wait who are you guys and what are you doing wandering around in the woods?
Sam: That's not important. We'll get you out of here but first you have to tell us if you've seen anything strange around here.
Dipper: HA! Have we? Where to start...well there were gnomes, the gobblewonker, shrinking crystals, ghosts, cursed wax figures...
Dean: Sounds like someone's got one hell of an imagination.
Dipper: No seriously! We even found this old creepy book that talks about it all. It's the town we live in. It has weird secrets.
Sam: What's your towns name?
Dipper: Gravity Falls.
Castiel snapped to attention and looked behind him as a screeching vorp sound started behind them and echoed throughout the woods.
Castiel: He's here...
A blue police box phased in and out of materialization as the wind picked up.
Dipper: No way...
Dipper flipped through his book to the sketch of the blue box as it finished materializing.
Sam: I'll be damned...
Mabel: Dipper it's the magic box!
Dipper: I know, I know!
Dipper yelled as he snapped his attention from the sketch to the Police box making sure it's real.
Sam: Either of you know how to open it?
Dipper: Well it does say one thing in here. Dipper showed the book to Sam and pointed to what he was referring to.
Sam: It's worth a shot.
The Doctor sat down in his chair still shaken about the dream he had.
The Doctor: It is returning through the darkness...what does it mean?
There was a set of four distinctive knock's on the TARDIS's door. The Doctor froze in fear. What if this were the four knock's Carmen mentioned? Who or what was behind that door that is so important that it could end time itself. The Doctor got up and calmly walked to the door. He put his hand on the handle and thought for a moment. He could just run, run and never look back. But what if Earth was at stake? What if the Universe was in peril? What if whoever knocked on his door needed help? The Doctor thought about it and then opened his door to see three men and two children. He walked outside and closed the door behind him.
The Doctor: Who knocked on my door?
Sam raised his hand.
Sam: That'd be me. Who are you and how'd you do that...trick of yours?
The Doctor didn't smile. He looked at Sam with the a depressed gaze.
The Doctor: I'm The Doctor.
Dean: Your real name.
The Doctor: I'm afraid that's all I've got.
Mabel: You're a Timelord aren't you?
The Doctor looked at Mabel in shock.
The Doctor: How did you know that?
Mabel: It's in his book!
Dipper held the book out to The Doctor who took it and read the section about Timelords.
The Doctor: Who wrote this?
Dipper: We don't know it's a mystery.
The Doctor handed the book back to Dipper. The Doctor: Very few people know that much about me. Even less are alive, but whoever they are they have a bad sense of humor if they thought that knocking four times would get my attention.
Dean: It worked didn't it?
The Doctor: Barely.
Castiel: Man of Gallifrey, you are what brought us here. But why?
The Doctor: I'm not sure, I like you lot. You're taking all the fun out of revealing things to you. No surprise, no mystery, no suspense! It's like you're all part of my fan club.
Castiel: Doctor! Why are we here?
The Doctor paused.
The Doctor: Time is ending...