The movie opens where Back to the Future left off: October 26, 1985. Marty (Michael J. Fox) opens the garage door and sees his 4X4, the one he longed for in Back to the Future. Jennifer Parker (now played by Elisabeth Shue) asks for a ride. Marty tells her that she is a sight for sore eyes, and wants to look at her. She says that he's acting like he hasn't seen her for a week. She asks if he's OK. When he sees his mom and dad, he says yeah.
As they are about to kiss, there are three sonic booms and the DeLorean appears and screeches to a stop in the driveway. Doc (Christopher Lloyd) jumps out and smashes into one of the trash cans. He frantically tells Marty that he has to come back with him. Marty sees his strange goggles and asks, "Where?" Doc answers, "Back to the future!" Doc rummages in the garbage can, and then grabs some banana peels and a discarded beer can. Marty asks him what he's doing, and Doc hastily claims he needs fuel.
He opens the attached Mr. Fusion Home Energy Reactor, and puts the banana peels in, topping it off with the beer. Doc tells Marty to get in the car, but Marty protests, saying he just got here, and Jennifer and he are going to take the new truck for a spin. Doc tells Marty that they'll take her along, because this concerns her too. Marty wonders what happens to them in the future, Doc hesitates for a moment, before replying that they come out fine, but that something has to be done about their kids.
After they get in, and Doc backs up into the street, Marty tells Doc that he had better back up, because they don't have enough road on the street to get up to 88 MPH. Doc quips, "Roads? Where we're going, we don't need roads." As Doc is starting up the car, Biff Tannen (Thomas F. Wilson) comes out of the house, and wants to show Marty the matchbooks he just got printed up for his auto detailing service, just in time to see the DeLorean fly away and vanish in a flash of light. "A flying DeLorean?" he asks, in disbelief.
They reappear in the year 2015, and immediately find themselves flying against traffic. Doc crosses to the right direction. They descend into Hill Valley on October 21, 2015. Jennifer is confused, and Marty tells her they're in a time machine. She asks about their future, the wedding, and where they will live. Doc knocks her out with a sleep inducing "alpha rhythm generator." He tells Marty that when she wakes up she'll think it was a dream. He says he did that because she was asking too many questions about her future, and she's not essential to his plans.
The DeLorean lands in an alley in the middle of a rainstorm. Doc tells Marty to wait five more seconds, and the rain stops right on the tick, as promised. Doc says they have an efficient weather service now, much more efficient than the postal service. When Marty gets out, Doc peels off a rubber mask, saying he went to a rejuvenation clinic and got a natural overhaul, including a spleen and colon replacement. He says they added 30 or 40 years to his life.Marty asks about his own future, if he'll be a rich rock star. Doc warns Marty that no one should know too much about their future. He tells Marty to change his clothes.
Meanwhile, Doc spies on Marty's future son Marty Jr. (also played by Michael J. Fox), saying they're precisely on schedule. The clothes Doc has given Marty have power laces on the shoes, and a self-adjusting fit on the jacket. Doc tells him to pull out his pants pockets, because all kids wear them like that today. He gives Marty a multi-colored cap like his son, then tells him to go into the Café '80s (one of those not-well-executed nostalgic-themed restaurants) in exactly two minutes and order a Pepsi.
A guy named Griff is going to show up and ask Marty whether he is in or out about "tonight's opportunity". Marty is to tell him he is out, say no, and come back immediately. Marty now asks what this has to do with his son. Doc shows Marty tomorrow's copy of USA Today, and Marty is shocked by what he reads: today, Martin McFly Jr. is going to be arrested for theft, and within 2 hours of his arrest, he will be tried, convicted, and sentenced to 15 years in the state penitentiary.
The justice system works swiftly these days now that all lawyers have been abolished. To make matters worse, next week, Marty's daughter Marlene (also Michael J. Fox, in drag) will attempt to break her brother out and will get sentenced to 20 years in jail, causing a chain reaction that destroys Marty's entire family. Doc's alarm watch is late, but he will try to intercept the real Marty Jr. before he can get to the Café 80s. Before Marty goes, Doc warns him to be careful because Griff has some short circuits in his bionic implants.
When Marty reaches the town square, he notices a small lake in the middle where the park, and parking lot, used to be. A car is in the robotic Texaco service station. The movie theater is showing "Jaws 19". Suddenly a hologram of a giant shark comes from the theater, swoops down on Marty and "bites" him, scaring him. He's not satisfied that the shark still looks fake. An electronic billboard advertises Wilson Hover Conversion Systems.
Marty sees an antique shop, where he notices unusual items like a B&W Macintosh computer, a lava lamp, and a Gray's Sports Almanac. The almanac lists sports results from the years 1950 to 2000. Marty goes into the Café '80s, which is full of nostalgic items from the 1980s. Several video screens are advertising the food & drink there; they are also the waiters. The screens use personalities from the 1980s: Michael Jackson, Ronald Reagan, and the Ayatollah Khomeini. They talk like Max Headroom.