The "Previously on Back to the Future" segment sets the scene, with the Doc returning to the current time, deciding he had given up on travelling time with his family, while Marty has had his first child with his girlfriend who he married, showing that he avoided the future shown to viewers in Back to the Future 2.
The first act opens with a frantic knocking on the door of the new McFly household, the kind of knocking that can only signify one thing: a time-travel experience. Marty stands up from the kitchen table, still wearing his familiar jacket and jeans, disregarding the fact he's now several years older for a marketing ploy wherein the jacket will be mass produced and sold as merchandise for the movie. He opens the door, and standing there is the one and only Doc Brown who blurts out, "Marty, we've got to go back to the future!" There is no explanation given as to why, as it is fairly clear to all that it would only be contrived and contradict the closure given in Back to the Future 2. Marty seemingly has no qualms with up and leaving his wife and child to travel time, despite the fact he has only encountered problems in his past endeavours doing so. The two run to the Delorean which is now parked outside the McFly residence and now cannot fly; another fact that is not explained. The familiar music starts up, and as the car hits 88 miles per hour, the lightning strikes and flames are all that are left in the present day.
Flash to 2020, where the car re-emerges in a future that looks exactly the same as our present, another contradiction to the previous movies. The car and our heroes begin slowing down as they hear a sickening thud and a scream, and the lifeless body of a small child is smashed against the windshield, which cracks under the force, and specks of blood manage to splatter against Marty and the Doc. Doc Brown slams on the brakes and the Delorean comes to a screeching halt; Marty is evidently shocked, visibly shaken and pale. He throws open the suicide doors, falls to his knees and covers his mouth with his hands as chunks of vomit find their way through the cracks between his fingers. Tears roll down his face as he screams at the Doc; "WE CAME HERE TO FIX THINGS, BUT WE KILLED A CHILD! WE ARE THE MONSTERS NOW! WE CAN'T FIX ANYTHING NOW!" The Doc is also pale due to the events, but manages to stutter, "It-it's okay. We can go back five minutes before this and... And prevent this tragedy from happening. We can fix things." Marty tries to stand up as the Doc puts the new time in the head unit of the Delorean. McFly stumbles to the car, wiping his tears away, and quietly says "I can't believe the damage we have caused..." He steps inside the Delorean, shuts the door, and speeds to 88 again, with an aim to avert the sickening events they themselves caused. However, in doing so, the Doc forgot to account for the time already spent in the future.The same events occur; the sickening thud, the lifeless body, the horrifying screams. Marty looks at the Doc and screams, "WE DID IT AGAIN! YOU SAID YOU'D FIX THIS, YOU WORTHLESS CUNT!" He grabs the Doc's labcoat and shakes him, his screaming now descended from his horrified thoughts to just primal, horrible howls of pain. Before the Doc can prevent his mistake, the Delorean from the first travel crashes into the back of our current heroes, destroying the Flux capacitor and leaving them stranded in the year 2020. Both pairs look at each other stunned; they have both taken the lives of a young child, and there's now nothing they can do to fix it. They sit in silence and await the arrival of the police, now understanding they deserve their fate and that there are consequences to their actions in the future.
Due to the brutal crime and recklessness of the perpetrators, there is a swift judgement on the case. Both pairs and the Delorean are to be imprisoned in a federal facility on a life sentence. The Delorean grows apart from the Doc and Marty, siding with white supremacists. Initially, Marty and Doc Brown go along with this, but after seeing that the white supremacists deal with other gangs in the facility, they cannot stand idly by. They voice their concerns and leave the group, bringing a swift revenge against Doc Emmett Brown, who is raped in the showers by the Delorean in a sickening homage to American History X and is left bleeding on the ground in the shower block.