On October 25, 1985, Marty McFly, a high school senior from Hill Valley, California, visits the home of his friend, eccentric local scientist Dr. Emmett L. "Doc" Brown. Finding Doc absent, Marty plugs his guitar into Doc's amplifier, but is blown across the room when its circuits overload. He then receives a call from Doc, telling to meet him at 1:15am at the parking lot of Twin Pines Mall. Their conversation is interrupted when Doc hears his clocks chiming 8:00 and excitedly tells Marty that they are exactly 25 minutes slow - which unfortunately means Marty is late for school. At school, Marty meets up with his girlfriend Jennifer Parker, and both are reprimanded for tardiness by school principal Mr. Strickland, who tells Marty that he is a "slacker" like his father, and sneers at the idea that his band are auditioning to play at the school dance, telling him that no McFly in the history of Hill Valley has ever amounted to anything. Marty defiantly tells Mr. Strickland that history is about to change.
Marty's band, the Pinheads, are rejected mere seconds into their audition for being "too darn loud". Marty confides in Jennifer that he doubts he has a future in music, and doesn't want to send his demo tape to a studio out of fear of rejection. Jennifer reminds him of Doc's advice to him: "If you put your mind to it, you can accomplish anything." As they talk, Marty admires a brand new Toyota pickup truck being delivered to Statler's Toyota dealership on the Courthouse Square. As he and Jennifer are about to kiss, they are interrupted by a lady soliciting donations for the Hill Valley Preservation Society, who oppose Mayor Goldie Wilson's plans to restore the courthouse clock tower, which was struck by lightning at 10:04pm on November 12, 1955. They take a flyer to get the woman to leave them alone, but then Jennifer's father pulls up, and before she leaves, she uses the flyer to write down her phone number as she is going to be at her grandma's house.
When Marty gets home, he finds that the family car, a 1979 Chevrolet Nova, has been wrecked by his father George's supervisor, Biff Tannen, who is berating George for not telling him about the car's 'blind spot' (the fact that Biff was drinking beer while driving may have been the real cause of the accident). He bullies George into doing his reports for him and dismisses Marty as a 'butthead' before leaving. At night the family are having dinner, and we learn that Marty's older brother Dave works at a Burger King (and has to ride the bus to work), while his sister Linda has no love life because their alcoholic mother Lorraine disapproves of girls chasing boys. Lorraine remarks that fate brought her and George together when her father hit George with his car after he mysteriously fell from a tree in front of her house. George then kissed her at the Enchantment Under the Sea dance, and that was when she decided to spend the rest of her life with him. Her nostalgic but dull tone as she tells the story makes it clear the romance has long since gone out of their marriage; George doesn't notice as he is too busy laughing awkwardly at the television like a dork.
Marty dozes off, but is woken up by a phone call from Doc around 12:30am. Doc asks him to bring his video camera to Twin Pines Mall. They meet up and while Marty starts to record his film, he learns that Doc has created a time machine out of a DeLorean DMC-12 and watches him perform a test run on his dog, Einstein, by sending him one minute into the future, a journey triggered when the car reaches 88 miles per hour. Overjoyed by this success, Doc demonstrates how the time machine works by entering several significant dates into the keypad. Doc enters the date November 5, 1955 and explains to Marty that on that day, he was hanging a clock over his toilet, slipped, and knocked himself out on the sink; when he came to, he had the idea for the flux capacitor, the device that makes time travel possible. Marty is more shocked that the DeLorean is nuclear-powered, as the only way Doc could find to generate the 1.21 gigawatts of electricity that powers the motor is to use nuclear fission. He admits to Marty that he got the plutonium by ripping off a group of Libyan terrorists who paid him to make a dirty bomb for them; he gave them an empty casing filled with pinball machine parts. But as Doc re-fuels the DeLorean and prepares to go into the future, the terrorists catch up with him and spray him with machine gun fire. Marty is able to escape in the DeLorean, but he accidentally activates the time circuits, and when he accelerates to 88 miles per hour in an attempt to outrun the terrorists, he inadvertently sends himself to the last date Doc entered into the keypad: November 5, 1955.
Twin Pines Mall was Otis Peabody's family farm in 1955, and Peabody and his family mistake the DeLorean for a crashed alien spacecraft and the radiation suit-wearing Marty for an extraterrestrial. As Marty flees the irate farmer's rifle fire in the car, he knocks over one of his twin pine trees. Marty tries to convince himself that he is dreaming, but as he returns to Lyon Estates, the subdivision where he lives with his family, he discovers that they have only just broken ground on the first houses. The DeLorean is out of power, so he hides it behind the large billboard advertising the glorious future awaiting families who live on Lyon Estates, then walks to the Courthouse Square, which is much cleaner and less run-down than it is in 1985. He goes to Lou's Cafe (occupied by an aerobics studio in 1985) to look up Doc Brown in the phone book, but when he calls the number, there is no answer, so Marty makes a note of Doc's address. He accidentally meets his father George, who is having breakfast at Lou's, when Biff and his friends bully him to copy his homework. After recognizing the janitor as future mayor Goldie Wilson (his offhand remark that Wilson will one day be mayor actually inspires him to pursue a career in local politics despite the racial barriers of the era), he follows George only to discover, to his horror, that his father is a peeping tom, hiding in a tree and using binoculars to spy on a girl undressing in a nearby house. Suddenly George loses his balance and falls into the path of an oncoming car; Marty instinctively shoves him out of the way and gets hit by the car instead while George flees.
Marty wakes up, convinced he's just had a nightmare about going back in time, but he bolts upright in horror when the teenage girl watching over him tells him he's back in "good old 1955", and is even more horrified to discover that the girl is his mother, Lorraine Baines. She assumes that his name is Calvin Klein since that is the name on his underwear, and she is clearly more than a little smitten by the mysterious boy who fell in front of her father's car. Marty joins Lorraine's family for dinner, meeting his then-infant uncle Joey, who cries whenever he is taken out of his crib (foreshadowing his future as a career criminal and jailbird), and confusing his uncle Milton when he claims to have seen the episode of The Honeymooners ("The Man from Space") that they are watching over dinner even though it is airing for the first time. Lorraine's mother Stella says Marty looks familiar, and asks if she knows his mother; Marty glances at Lorraine and says she probably does. When Lorraine begins aggressively flirting with him, he quickly makes excuses to leave in search of Doc Brown; Lorraine's unimpressed father Sam says that Marty is an idiot who must have idiot parents, and that if Lorraine has a child like that, he'll disown her.
Marty shows up at Doc's mansion (which had been blown up by a lab accident by 1985), but 1955 Doc refuses to believe that he is from the future; he says that Marty's picture of himself and his siblings (in which Linda is wearing a "Class of 1984" sweatshirt) is shoddy trick photography, as the top of his brother Dave's hair has been cut off, and finds the idea that B-movie actor Ronald Reagan is US President in 1985 to be ludicrous. But when Marty tells him that he knows how Doc got the bruise on his forehead - from hitting his head against the bathroom sink and conceiving the flux capacitor, the device that makes time travel possible - he finally convinces him. When Marty shows Doc the DeLorean and the flux capacitor, which looks just like Doc's drawing, Doc is overjoyed to discover that he has finally invented something that works. He watches the video Marty recorded in 1985, but is horrified to discover that the DeLorean needs 1.21 gigawatts of electricity to work, and he remarks that while he is sure plutonium is easy to find in 1985, it's far rarer in 1955, and the only power source they can use is a bolt of lightning - but there's no way to know where or when one will strike. Marty remembers the flyer about the clock tower and shows Doc that lightning will strike the clock tower the following Saturday at 10:04pm. Doc is delighted again, but hurriedly shoots down Marty's suggestion that he can show him around Hill Valley, as anything he does while in 1955 might irreversibly and disastrously alter the future. Marty awkwardly admits that he has already run into his parents, and Doc asks to see the picture of the McFly siblings again - and discovers that now Dave's entire head has vanished, erased from existence.
Doc takes Marty to Hill Valley High (which is much cleaner than it is in 1985, although Mr. Strickland is still the school disciplinarian and was bald even then), explaining that he needs to make his parents fall in love before he can go back to 1985. Marty admits that his parents' relationship has never made sense to him, as they have nothing in common. He sees George in the hallway between classes and tries to introduce him to Lorraine; she, however, is too smitten by Marty to even notice George. At lunch, Marty is surprised to discover that George writes science fiction short stories, but has never let anyone read them or submitted them to publishers for fear of rejection. Marty tries to persuade George to ask Lorraine to the Enchantment Under the Sea dance, but George refuses for further fear of rejection and because she's probably already going with Biff, who is aggressively hitting on her. Marty pulls Biff off Lorraine, and only the timely appearance of Mr. Strickland stops Biff from beating him up in retaliation. After school, Marty chases after George, who is adamant that he is not going to the dance as he doesn't want to miss Science Fiction Theater, and says "no one on the planet" can persuade him, which gives Marty an idea. That night, he dresses in his radiation suit, wakes George by putting headphones on his ears and blasting guitar music through them, then introduces himself as "Darth Vader... from the planet Vulcan" and threatens to fry George's brains if he doesn't ask Lorraine to the dance.
The next morning, George finds Marty and asks for his help in wooing Lorraine. Marty agrees, but suggests he keep quiet about "Darth Vader" as no-one will believe him. In Lou's Cafe, George attempts to flirt with Lorraine and, though fumbling the words due to nerves, he seems to be getting somewhere when Biff shows up and prepares to throw him out. Marty trips Biff before he can get to George, then distracts him for long enough to land the first punch before fleeing to the Courthouse Square, where he grabs a young boy's box trolley and snaps off the top to create a makeshift skateboard. Biff and his gang drive after him in their Ford and eventually pin him to the front of the car, but he outsmarts them by climbing over the car and hopping onto his skateboard as it appears from behind it; the bullies are so stunned, they do not notice they are about to crash into the back of a manure truck until it is too late. Unfortunately, this display of heroism makes Lorraine more attracted to Marty than ever.
Marty watches Doc demonstrate how he plans to get him back to 1985 using a model of the Courthouse Square, but his fascination gives way to alarm when he discovers that Lorraine has followed him back to Doc's mansion to ask him to the Enchantment Under the Sea dance. Since Lorraine tells Marty that she is attracted to strong people who protect those they love, he decides to set up a scenario that will make it seem as though he is trying to take advantage of Lorraine before George comes to her rescue. George warms to the idea and tells Marty he wishes he had his courage. Remembering what Jennifer told him about his music career, Marty tells George that he just needs self-confidence: "If you put your mind to it, you can accomplish anything." Marty borrows Doc's car for the date, and as Doc makes the final preparations to send Marty back to 1985, he says he'll miss him after he goes back, as he feels his life is better for having met Marty in 1955, especially now that he knows he has the experience of time travel to look forward to. Distraught over the thought that Doc will not live to use his greatest invention, Marty tries to warn Doc about the terrorists, but Doc refuses to learn anything about his future for fear of disturbing the space-time continuum. Marty writes Doc a letter urging him to take any precautions necessary to prevent his death at the hands of the terrorists and slips it into his jacket pocket.
On the night of the dance, Marty is stunned to discover that Lorraine has swiped liquor from her mother's drinks cabinet and smokes cigarettes, and has previous experience fooling around with boys in parked cars. However, when she aggressively kisses Marty, she withdraws, saying it's like kissing her brother, a reaction Marty finds more relieving than surprising. Just then, the car door opens - but Marty finds not George, but Biff, who is drunk and furious with Marty over the manure incident. He shuts himself in the car with Lorraine and tells his gang members to take care of Marty; they lock him in the trunk of a car which turns out to belong to the band for the dance, Marvin Berry and the Starlighters, who are outside for a smoke break and chase Biff's gang away. Unfortunately, the car keys are locked in the trunk with Marty.
George shows up at Doc's car on cue and tries to follow the script of his plan with Marty, but is alarmed to discover Biff in the car instead of Marty. Biff tells him to leave, but George refuses, so Biff twists his arm behind his back. When Lorraine tries to intervene, Biff shoves her to the ground and laughs. The enraged George finally discovers his self-confidence and knocks Biff out with a single punch to the jaw. Marvin finally gets Marty out of the trunk of his car by jimmying the lock with a screwdriver, but cuts his hand in the process. Marty arrives just in time to see George and Lorraine head into the dance together, but when he checks the photograph, Dave is completely gone and Linda is rapidly disappearing too. Marvin's injury means the band cannot play on, and so George and Lorraine's first kiss cannot happen - unless Marty knows someone else who can play guitar...
Marty joins the Starlighters on stage both to play guitar and to keep an eye on George and Lorraine, but during the first number, "Earth Angel", Mark Dixon, a student, cuts in between George and Lorraine. Lorraine tries to get George to intervene, but he has lost his confidence again, and Marty collapses on stage as he begins fading out of the photograph and out of existence, his hands becoming semi-transparent. Fortunately, George's crisis of confidence proves short-lived as he shoves Dixon to the floor and kisses Lorraine passionately. Marty springs to his feet and sees himself, Linda, and Dave re-appear in the photograph; overjoyed, he begins to play the guitar with renewed vigor. At the band's request, Marty plays one more song, "Johnny B. Goode". Marvin Berry calls his cousin, Chuck, and tells that he found the "new sound" Chuck was looking for. Marty does Chuck Berry's trademark duck walk, and then gets carried away imitating other guitar heroes: windmilling his arm and kicking over his amplifier in imitation of Pete Townshend, lying on the stage kicking his legs in imitation of Angus Young, playing behind his head like Jimi Hendrix, and tapping in the style of Eddie Van Halen. In the face of uncomprehending stares from the audience, Marty says, "I guess you guys aren't ready for that yet. But your kids are gonna love it."
As Marty leaves the dance, he runs into Lorraine and George one last time. Lorraine tells Marty that George has asked to walk her home; far from being upset, Marty is delighted, telling them he has a good feeling about their future together. When they ask if they'll ever see him again, he tells them, "I guarantee it." Before he leaves, he tells them that if they have a child who, when he is eight years old, accidentally sets fire to the living room rug, "Go easy on him."
He rejoins Doc Brown, who has suspended a cable from the top of the clock tower to channel the lightning into the DeLorean; Doc seems uncertain how to react to the news of George's bravery, which even Marty admits was completely unlike him. Just as Marty is getting ready to take the DeLorean to the starting line Doc drew, Doc discovers Marty's letter in his jacket pocket and, refusing to learn anything about his future for fear of disturbing the space-time continuum, begins tearing it up, but before he can throw the pieces away, a tree branch falls onto the cable, disconnecting it. Doc absently puts the pieces back in his pocket and goes to the top of the clock tower to re-connect the cable; Marty tries to yell his warning about the terrorists, but is drowned out by the clock striking ten. With only four minutes to go before the lightning strikes, Marty has to rush to the DeLorean, and decides to simply go back to eleven minutes before he left so that he can get to the mall in time to warn Doc of his fate - but when he tries to start the car, it goes dead. Meanwhile, Doc is poised to plug in the cable at the clock tower end, but it is pulled so tight by the tree branch that it unplugs at the street level end. Just as the DeLorean finally roars into life and Marty speeds down the road, Doc plugs the cable in at the clock tower end and slides, zipline style, down the cable and re-connects it at street level mere seconds before the lightning strikes and sends Marty and the DeLorean back to 1985. The ecstatic Doc runs down the fire trail left by the DeLorean, laughing with delight at his successful experiment before looking toward the clock tower.
Marty returns to 1985, but the DeLorean has stopped working again, and while Marty frantically tries to start it, he sees the terrorists' VW bus drive past on its way to Lone Pine Mall (as it is now known since Marty ran over one of the twin pines). Marty arrives just in time to see Doc get shot as his past self screams in grief before outdriving the Libyans and going back in time while they crash into a Fox Photo booth. Marty rushes down to Doc's body and turns away in tears. Suddenly, Doc wakes up and reveals to Marty that he has a bulletproof vest under his radiation suit. Marty asks how he knew, and Doc shows him the letter from 1955, yellowed with age and taped back together. When Marty jokingly asks about disturbing the space-time continuum, Doc tells him, "Well, I figured 'what the hell?'". He drops Marty off at his house and drives off to 2015 with Einstein in the DeLorean.
Marty wakes up the next morning, convinced his time travel adventure was just a nightmare, but when he goes into the dining room for breakfast, he is amazed to discover the house is much more tastefully and expensively furnished, that Linda has so many prospective boyfriends that even she can't keep track of them, and that Dave has an office job. He is even more amazed to see George and Lorraine return from a tennis game looking fit and happy, and clearly still very much in love. Lorraine is now much more approving of Marty's relationship with Jennifer, but when Marty reminds her that the wrecked car has derailed his weekend plans with Jennifer, the family are shocked. George assures them that the car is just fine, and opens the door to reveal that not only is the McFly family car a brand new BMW 733i, but Biff is outside waxing it in his capacity as an auto detailer. George even catches Biff in a fib about whether or not he has put on the two coats of wax for which George has paid him, revealing that Biff no longer feels able to bully George. Both George and Lorraine agree that for all his faults, Biff is the one who caused them to fall in love. Just then, Biff enters with a delivery box containing the proofs of George's first novel, a science-fiction story called A Match Made in Space. George proudly echoes Marty's advice to him in 1955: "If you put your mind to it, you can accomplish anything," advice he has taken to heart to become a successful writer. Biff also gives Marty his new keys, which he discovers are for the Toyota pickup he was admiring in the original timeline.
Jennifer arrives as Marty takes in the sight of his new truck, and they decide to go for a ride together. Just then, Doc reappears in the DeLorean, and urges Marty and Jennifer to return to 2015 with him, as something needs to be done about their kids. After Doc re-fuels the now fusion-powered DeLorean with some of the contents of the McFlys' garbage can, Marty and Jennifer enter the car and Doc pulls out of the driveway. Marty points out that there is not enough road to accelerate to 88 mph. Doc replies, "Roads? Where we're going, we don't need roads," and flies off in the now hover-converted DeLorean.