Marty froze in place, staring at the old scientist. "You... you..." he managed to say.
His face twisted in a grimace. He clenched his fists and his teeth. Finally, he threw himself at Doc and grabbed him by the shirt. "You bastard!" he screamed.

Doc grabbed Marty's wrists. "Calm down, Marty." he said. "All I did was in good faith. I did it to save the world, and what's more important, I succeeded."

Marty released his hold.
"If you just sit down and let me tell you exactly what I did, you'll understand everything." continued Doc.

Marty stepped back and went to sit in one of Doc's armchairs. Doc and Jennifer did the same.

"It all began in 1897. My time train... at least, the first version, was complete. Let's call it Time Train 1.0." started Doc.
"The capacitors had taken two years to charge, and I knew they would not last more than one trip, so I had to make it count. The destination time I chose was January 1, 2035. Precisely 150 years after I reached the Old West.

As I predicted, when I got there, the capacitors were FUBAR. That was not a problem, because by 2035, fusion and hover technology were so advanced, they could make a train fly and even make a flux capacitor work for multiple trips without refueling. I made extensive repairs, and when I was done, the train looked like what you saw that Sunday morning with Jennifer. Let's call it Time Train 2.0.

I used Time Train 2.0 to reach 2135, and what I found... was a wasteland. A neverending winter enveloped the whole planet, electronic technology was a distant memory and people lived as scavengers in nomadic tribes, seeking shelter in what remained of devastated cities. From generation to generation, they recounted tales of a lost utopia and an event they called the Great Disaster, caused by an individual they called the Destroyer of the World."

"So what did you do?" asked Marty.

"I needed to prepare a plan of action" said Doc. "I considered the possibility that, in spite of my efforts, I would fail and die, so, first, I came to say hi to you and Jennifer with my wife and children, then I took them safely back to 1897, and finally I returned to 2035. From there, I moved forward, one year at a time, until I pinpointed the year of the Great Disaster as 2045."

"Did you go there?" asked Marty.

"Yes" said Doc. "And I saw the rise of a new means of communication. An open source information space connecting all sorts of devices from computers, to phones, to televisions, to energy generators. The World Wide Web."

Marty was about to say something, but Doc continued. "And then... on October 21, I saw a nuclear holocaust. I saw a hundred million Mister Fusion devices explode at the same time all over the world. I saw four billion people die instantly. I travelled back and forth, trying to discover what had triggered the explosions. And then I discovered the identity of the Destroyer of the World. Griff Tannen."

"That's impossible!" Marty was in complete disbelief. "Griff has cerebral palsy! You know the rumors, right? Biff impregnating his own daughter? Even in that other timeline, Griff sounded like he was... special."

"Indeed, Marty." nodded Doc.
"But in most cases of cerebral palsy, there are no learning disabilities. When Griff was in prison, with nothing to do, he started to study, and when he got out, he started a software company. Grifftech. On October 21, 2045, when the dot-com bubble was in its full force, he released a virus from a Grifftech computer. The virus infected all Mister Fusion devices and caused them to explode.

That's when all became clear to me. I knew what I had to do. I flew all the way to Salt Lake City and reached March 10, 1989. The fathers of cold fusion, Martin Fleischmann and Stanley Pons, were running their experiments at the University of Utah. I made sure those experiments would fail! I replaced their equipment with flawed copies, and I deliberately introduced mistakes in their notes. It worked! Time Train 2.0 disappeared before my eyes, replaced by the plutonium-powered version of the Delorean. Soon, scientist groups all over the world announced their inability to reproduce their results, and in less than a year, cold fusion was dead.

However, I didn't mean to halt progress completely. I reached Geneva, Switzerland, and met with a young computer scientist named Tim Berners-Lee, who was an employee of CERN. I gave him the World Wide Web specifications I took from 2045, and assisted him in implementing them, fifty-six years earlier.

So, you see Marty, I may have prevented an energy revolution, but I kickstarted a computer revolution! Every time you connect to the Internet, you use technology that was not supposed to exist for thirty more years!"

Marty could not believe his ears and sat there, in silence.
In his mind, he thought of all the time he had spent wondering why the timeline was deviating so much from what he saw, all the time he had spent hoping that somehow it would fix itself before that fateful date of October 21, 2015. As it turned out, the culprit of everything was his best friend.

Marty was still mad with rage, and stared at Doc with bloodshot eyes. He stood up from his armchair.
"ARE YOU OUT OF YOUR FUCKING MIND?" he finally thundered.