Three days later - or twenty-six years before - Marty was approaching the Department of Physics at the University of Utah, wondering which door Doc would use. Then he remembered a piece of advice from Doc himself: "It's no use to sneak around. Move in plain sight, and make it look like you have a reason to be there." As a teenager, Marty had followed it regularly whenever he was late for school.

Marty opened the main door, walked in, and remained next to it, waiting.
Because that was the crucial part of the mission, Marty had deliberately taken less medications than usual: a stiff gait would attract less attention than uncontrollable movements.

After two and a half hours, a man with a wild white mane, dressed in a sober gray suit, came in through the door. Marty pulled something out of his pocket, and when Doc walked next to him, hit him in the abdomen.
Doc immediately stopped, bent over in pain, and fell on the floor. He put a hand to his midsection and looked at it. He was bleeding.

Marty approached him and crouched. "Hi, Doc" he said slowly. "Trying to kill cold fusion, are you?"
He showed the bloody Swiss Army knife he was holding in his hand. "You're not the only killer here." he added.
Doc coughed. "M... Marty." he said, in a whisper. "How do you know?"
"You told me, in a version of 2015 that's not supposed to exist." answered Marty.
"I must do it... please help me..." whispered Doc. He coughed again.
"I won't let you become the Destroyer of the World." said Marty, and plunged his knife right into Doc's heart.
Doc let out an atrocious scream, while blood poured out of his body.

With the last of his strength, Doc tried to utter a final word. "P... Pa..."
One last cough. "Para... dox."
Doc ceased to move. Only then Marty stood up and walked away.

While Marty was returning to the Delorean, the air around him felt weird. It seemed to vibrate and transmit vibrations to his skin. In the distance, a soft rumble was gradually increasing in intensity.
He looked up, expecting to see a plane, and what he saw froze the blood in his veins. It was as though the sky was a surface made out of pointlike particles, which were now coming apart and revealing a dark void beyond, blacker than any black he had ever seen.
Around him, the same thing happened to the buildings, trees and streets, while the rumble rose to a deafening noise.

Marty looked down at his own body, and was horrified to see it was coming apart too.
In a flash of intuition, he realized what was happening. In 2015, Doc had told him how he had changed the timeline in 1989. Then, Marty had reached 1989 and killed Doc, preventing those changes from happening. But then, 1989 Doc would never become 2015 Doc and would never tell Marty of those changes. Marty would never reach 1989, so Doc would make those changes and tell Marty about them in 2015. Marty would then reach 1989 and...
It was the classic grandfather paradox, which was now destroying the universe, just as Doc had once predicted. In an instant, all matter tore itself apart.

Elementary particles, now chaotically mixed together, formed a uniform, but unstable, hot plasma, which collapsed under its own gravity, becoming a gargantuan black hole.
The energy density was too great to be sustained, and the black hole exploded into a new Big Bang.
The four fundamental forces separated and gave birth to fermions and bosons. What would happen from there was nobody's concern, because for thirteen billion years, nobody would be there to care.