A/N: I'm not entirely sure where the inspiration for this came from, but hopefully it'll turn out okay. This is set in the normal Portal 2 universe, but where everything with Chell never happened, and Marty and Doc were the ones to go through that instead. Bit of a weird crossover, but I love Portal and I love BTTF, so this happened!

...

"So, Doc, what's the plan?" Marty leaned against the DeLorean casually, arms crossed. "What's this thing supposed to do?"

"Theoretically," Doc replied, "It should allow us to travel much further through time than before. The previous limit on the time circuits was December 31 9999 at 11:59pm. This should allow us to travel much further around,"

Marty nodded and continued watching as the scientist tightened screws and adjusted knobs and whatever else he was doing to the contraption. It all looked pretty confusing; the workshop was a mess of wires and bolts, pieces of metal were everywhere and papers littered the floor. Alright, the papers were normal, but the rest seemed out of place. Einstein was dozing in a corner, his bed partially covered by the papers.

"How long do you think this will take?" Marty asked after a while.

"Shouldn't be too long," Doc responded, tongue poking out in concentration. "I just have to make sure all these damn wires have a place to go. I can't possibly leave them like this - it would be far too dangerous."

"Right, not to mention it would look pretty half-hearted," Marty added.

"I guess it would match the rest of the car then," Doc chuckled. "I never could be bothered to make it look particularly presentable. As long as it works, I don't really care what it looks like,"

Marty laughed. "I guess it looks cooler that way. It doesn't make it look like it's been made in a factory."

"The initial parts were made in a factory," the scientist pointed out. "But the time-travel components were all handmade. You know that,"

"Exactly," Marty replied. He leant his head back and yawned. He hadn't been sleeping particularly well recently on account of the fact that he was under a lot of stress from his final exams. He'd almost flunked History so he was determined to do better in the rest of them.

Doc looked over to him and shook his head. He knew Marty had a tendency to leave things until the last minute, which is what he'd been doing with his exams recently. He'd been studying like mad to try and cram in a year's worth of knowledge in a few days, meaning staying up until the early hours every night so far. He just hoped that Marty would be alright in his exams.

As he finished tightening the last bolt, he put the wrench down. "Come on, let's go get you some coffee,"

"Thanks," Marty yawned again before standing up straight, beginning to follow the scientist out the door. "I need it,"

"You need to stop pushing yourself so much," Doc replied. "This is why it's best to start revising early, so you don't land in this situation,"

Marty rolled his eyes. "It's a bit late now, isn't it?"

Doc sighed. "I still told you to start revising early, didn't I?"

"Yeah, yeah," the teenager sighed.

Doc rolled his eyes as he pushed his front door open. Jules and Verne were having a competition on one of the 21st-Century consoles their father had bought for them. It looked like some sort of racing game, but Marty wasn't paying any particular attention. Clara looked up from where she was reading at the kitchen table as they entered.

"How's the DeLorean going?" she asked, replacing her bookmark and putting the book down.

"Those wires are still being a nuisance," Doc replied, taking a seat opposite her at the table. "And Marty's yawning doesn't make it any easier,"

Marty took a third seat to Clara and put his arms on the table, laying his chin on them. "Sorry, I've just been insanely tired recently."

"If you'd started your revision earlier, you wouldn't have to cram it all into a couple of weeks." Doc replied, rolling his eyes.

Marty groaned and buried his whole face in his arms. Clara laughed. "He's right you know, Marty."

"That's what's making me wish he'd shut up," the teenager groaned.

Doc shrugged and sat back in his seat. "It's your own fault,"

Marty groaned loudly again. "You sound just like my parents!"

"Maybe that's a good thing, that way you have two pairs of adults trying to talk some sense into you," Doc smirked. Clara laughed again.

Marty shot Doc a glare. "I came round to help you work on the DeLorean, not to be lectured about school,"

Doc laughed and elbowed his shoulder gently. "Well go and get some coffee then meet me back out in the garage,"

Marty rose from the table and walked over to the coffee machine, placing a mug on the worktop next to it. A minute later, he was holding a cup of strong coffee in his hands, taking a sip from it every now and then. He followed the scientist back out to the garage and set back to work on the DeLorean.

The pair worked for another hour, during which time Marty completely drained his coffee and had gone back for a second mug, before the device was fitted to the dashboard and the newly-modified time machine was ready for testing.

"Come on," Doc gestured for Marty to join him inside. "We'll go ten thousand years into the future and see what happens,"

Marty gave the car a wary look. "Are you sure this is a good idea? I mean, we've only just built the thing. When you first tested out the DeLorean you used Einstein instead,"

"But that was only a minute into the future, and Einstein can't drive the DeLorean by himself." Doc pointed out. "At least we can get back to our original time,"

Marty sighed and got in, strapping the seat belt on. "You're the doc, Doc,"

Doc took this as the signal and started up the DeLorean's engine. He drove it out onto an open stretch of road. It was absolutely pouring with rain and Marty could have sworn he heard thunder.

"Hey Doc? Can't this wait until later? Remember what happened in 1955, last time there was lightning around?"

"All too well," Doc shuddered a little. "But you must remember, lightning is attracted to the nearest point on the Earth's surface, so it would most likely strike something tall like a tree or a building. The DeLorean is only five foot tall, there's practically no chance of us being struck,"

"Right," Marty still didn't sound too confident, but remained in the seat. "Let's get this over with,"

Doc nodded and pressed down on the accelerator. The car began to gain speed, and he turned the time circuits on. The speedometer gradually crept closer to eighty eight.

Just as that number hit, so did something else. And that something else was a bolt of lightning. The sparks engulfed the car in an ear-splitting shout, drowning out Marty's scream as the car vanished.

While the DeLorean was indeed only five foot high, it was made of steel, an excellent conductor of electricity. The lightning had been attracted to it, struck the car and taking its two occupants far into the future.

…

When the car eventually rolled to a halt, both occupants got out. The ice covering the car melted, then evaporated easily in the blaring midday sun. The landscape was bleak, to put it lightly. Tall, brown grass lay around them for miles and miles in every direction.. The flat horizon was punctuated only by a small, wooden shed placed perhaps a hundred feet from where they were currently. Marty could see ruins of old buildings, clearly Hill Valley, resting in the grass.

"Jesus Christ Doc! Where are we? When are we?"

Doc looked at the display on the dashboard of the car. The display was fizzling, displaying seemingly random numbers, none of which remained there for very long. They had been short-circuited by the lightning. "I don't have a clue. The display is busted,"

"Damnit," Marty cursed. "Well can't we just go back?"

"No," Doc sighed. "It seems as though both the time circuits and the flux capacitor are completely fried,"

Marty groaned. "You have got to be kidding me,"

"I wish I was, Marty," the scientist sighed. "Come on, maybe there's something in that shed we can use,"

"Right," Marty nodded glumly, beginning to walk over to the small shack. Doc followed him, but he couldn't help but look around at the scenery. From the looks of the rubble, something had burned the whole of Hill Valley to the ground a long time ago. All he could see were some broken rocks, twisted metal bars and … was that a skeleton?!

Doc shuddered and returned his attention to the shack. Several warning signs were painted on the door, reading "CAUTION". That couldn't be a good thing. Hanging above the door, to the left, was a small security camera, although it looked more like a white eyeball than any camera Doc had ever seen. He had a feeling that this shed was more than it appeared to be.

Marty pulled the door open and was met with a white, cylindrical elevator. It appeared to be very well made, with its glass sliding door wide open. He narrowed his eyes in suspicion. Why would an elevator be placed in a tiny shed in the middle of nowhere? Had it not been for Doc being there too, he would have been dead in three seconds.

Standing in the corner was a three foot high, white, slightly oval-shaped object. It had a red circle painted on the front of it and it stood on three legs. A red laser beam came from its optic, if that is what the circle could be called. At the presence of the two newcomers, the sides of object pulled apart, revealing two guns on either side. The red laser beam drifted over towards them, landing firmly on Marty's chest.

"There you are…"

Doc grabbed Marty and pulled him backwards out the door. The scientist tripped over and the teenager fell backwards into his lap. Several loud shots could be heard, before that high-pitched voice said "Sentry mode activated," and the red laser beam stopped drifting around.

"What was that?" Marty exclaimed. "Was it trying to kill us?!"

"It appears so," Doc replied, panting a little bit. He still had a firm grip on Marty's arm. "Are you alright?"

"Y-yeah, I'm fine," Marty got to his feet, brushing himself off and offering Doc a hand up. He pulled the scientist to his feet. "You?"

"Just a little startled," Doc replied. He looked back at the DeLorean, then to the shed again. "Our only hope of getting home is checking inside there, but we'll have to be quick."

Marty looked at the shed warily. "Alright…"

Both of them took a deep breath and ran straight for the elevator. The white thing barely had time to target them before the door automatically slid shut and the tube descended deep into the ground. The elevator was pretty small, and Marty was pressed against Doc's chest for the whole journey downwards. If either of them had claustrophobia, this would be a nightmare.

As they descended, Marty caught glimpses of other rooms. Some looked as though there were overgrown with ivy and other plants, others were full of more of those white objects he'd seen earlier. Sometimes vast, empty chasms could be seen through the glass, fog preventing them from seeing further into the underground labyrinth.

Soon, the lift came to a stop. The door slid open and Marty walked out, soon followed by Doc. The room they were in at the moment was bleak and bare. The walls had partially crumbled away and vines grew in from the sides. Dust covered the floor and the sound of dripping water could be heard from somewhere beyond the wall.

A set of metal steps led up to a circular door with a blue stick figure painted upon it. It looked as though the figure was running out from behind a wall. The pair walked up the steps, their footsteps echoing around the room. The door slid open, revealing a metal walkway towards what looked like a set of hotel rooms. They too looked abandoned. A metal rail hung above their heads, reminding Marty of train tracks. One of the doors was left open and, cautiously, both of them decided to walk into the small room.

They were met with a bed and some rather withered furniture. Blinds on the windows let in a thin shaft of light, indicating that the room was rather dusty. As they took a few steps further into the room, a draught blew and the door slammed shut. Doc would have pulled on the handle, had there been a handle. The pair of them were now stuck there. The blinds seemed to automatically slide shut, making the room a lot darker than it had been a minute ago.

"Now what?" Marty asked, feeling Doc place a hand on his shoulder.

"I think we should just sit down and wait this out," Doc replied. "The worst thing to do would be to panic,"

Marty let out a breath. "Right…" He sat on the floor with his back against the bed and Doc sat next to him. After a little while, Doc began to drift off. He slumped down, snoring. Marty soon joined him, falling asleep.