In the face of uncomprehending stares from the audience, while lost in heavy metal riffing, Marty stops and tells the students "I guess you're not ready for *that*. But your kids are gonna LOVE it." Marty turns to leave the dance and runs into George and Lorraine. Lorraine asks if it's OK for George to take her home and Marty heartily agrees. He also advises them that if they have a son who accidentally sets fire to the living room rug when he's eight years old, to go easy on him, implying that he's talking about himself.
At the town square, Doc waits impatiently for Marty. Marty arrives, saying he needed time to change back into his 1985 clothes. As they prepare for the event, Doc discovers the note from Marty in his pocket. Refusing to know too much about his future, he tears up the note without reading it. Just then, a falling tree limb disconnects the cable he has installed from the clock tower to the street.
Doc climbs again to the clock tower and has Marty feed him the cable. Marty also tries to warn Doc about his death but is drowned out by thunder. Marty runs back to the DeLorean and races off to the starting point Doc has painted for him. While waiting for the timer to go off, he resets his destination time to arrive 11 minutes earlier than he left so he can warn Doc.
Just then, the car stalls and Marty frantically tries to start it again. When it does restart, after the timer goes off, Marty begins speeding toward the town square. Despite some difficulty, Doc reconnects the cable just as the lighting bolt surges through the line and the DeLorean speeds off into the future, leaving behind a pair of fire trails. Doc celebrates joyously in the street.
Back in 1985, around 1:19 AM, a homeless bum is seen sleeping on a bench in the town square when he is woken up by three sonic booms, just as the DeLorean materializes and slams into a storefront just down the block. The bum, Red, quips "Crazy drunk drivers." Marty backs the DeLorean out and turns around, only for the car to promptly ice up from the time travel trip. Just then, the Libyans' blue VW minibus passes by, driving recklessly. Marty is forced to run to the mall where the initial experiment is taking place.
As he arrives, we see that the sign now reads "Lone Pine Mall," another indication of how Marty has accidentally altered the past. He sees Doc get shot, again and watches from a distance as the Libyans chase his previous self around the parking lot. When the DeLorean vanishes and Marty's counterpart goes back to 1955, the Libyans lose control of their van, which tips on its side and crashes into the photo kiosk. Marty runs down to Doc. Marty is devastated that he couldn't arrive in time to save Doc.
Doc, however, suddenly, bolts up. Marty is stunned until Doc opens his shirt, revealing that he's wearing a bulletproof vest. Marty asks him about the consequences of changing the future and the space-time continuum and the Doc admits, "Well, I thought, 'What the hell!'". Doc drives Marty home and tells him he plans to venture about 30 years into the future. He then ramps the DeLorean up to 88 mph and drives off into the night in a flash of light.
Marty wakes up the next morning to find that the furniture in his house is arranged differently. Dave is wearing a suit and working for an accounting job. Linda seems to be having trouble keeping track of all the teenaged boys who keep calling her for dates, much to Dave's exasperation. George and Lorraine arrive home from a tennis match, happy and even a bit frisky. Lorraine asks Marty about the camping trip he has planned with Jennifer, to which Marty mentions that the car is wrecked.
Everyone starts barking about it until George shows them that Biff is waxing the car in the driveway. Biff now runs an auto detailing service and now is working for George, rather than the other way around. George seems amused at Biff's efforts to get away with as little work as possible (but now confronts Biff to complete the work he was hired for). Moments later, Biff comes into the house carrying a box filled with copies of George's first published book, the cover of which resembles Marty's appearance in the radiation suit.
Marty is unsure how to take everything in when Biff hands him a set of keys. They are for the Toyota pickup truck he'd been thinking about purchasing with Jennifer back at the beginning of the movie. As Marty goes into the garage and looks at the truck, Jennifer appears behind Marty and he's relieved to see her and happy that his family is happier as well. Suddenly there is a burst of electricity and the DeLorean screeches to a halt.
Doc gets out, dressed in wild clothing and tells Marty he needs to come with him to the future; something is wrong with his and Jennifer's kids. Doc gathers "fuel" by rummaging through a garbage can and loads it into a new addition to the car's engine called Mr. Fusion. All three pile into the DeLorean and it backs out of the driveway. Marty tells the Doc he needs to back up further to get up to 88 mph, as they have no road. Doc replies, "Roads? Where we're going, we don't need roads!" Doc has converted the car to a hovercraft. The car takes off, and flies at the camera.