On October 25, 1985, Marty McFly, a 17-year old high school senior, visits the home of his friend, an eccentric local scientist named Dr. Emmett L. Brown, but finds that "Doc" is not there. Moments before Marty had walked in, a report came across the television about missing and stolen plutonium. As Marty walks into Docs house, he kicks his skateboard and it runs into a box of plutonium that is under Docs bed. Marty, however, does not notice the box and proceeds to hook his guitar up to the giant amplifier that Doc has constructed. Marty turns the amplifier settings to their highest points and strums his guitar. The amp blows up and sends Marty flying into the shelves behind him. Marty soon after receives a call from Doc asking him to meet him at 1:15 AM in the parking lot at Twin Pines Mall. As Marty agrees, the clocks in Doc's basement chime the hour. When Doc Brown remarks that the clocks are 25 minutes behind, proof that an experiment was successful, Marty realizes that he is late for school. Winnie the Pooh, Piglet, Tigger, Rabbit, and Eeyore have arrived in Hill Valley, but see Marty running late for school. Rabbit decides that he and the others should follow Marty so they would meet him.
When Marty gets to school, he is found by his girlfriend Jennifer Parker. She informs Marty that Mr. Strickland is looking for him. Mr. Strickland finds the kids and gives Jennifer a tardy slip while reprimanding Marty. After school, Marty and his band, "The Pinheads," audition to play at the school dance, but the band is rejected for being "too darn loud". Afterward, Marty confides in Jennifer, about worrying that he will never get a chance to play for an audience. As they sit on a bench in the Courthouse square of Hill Valley, they are interrupted by a woman insisting that Marty and Jennifer help "save the clock tower." The lady proceeds to hand Marty a flyer about a campaign to save the clock tower, which was struck by lightning at 10:04 PM on Saturday, November 12, 1955. Jennifer soon has to leave and writes her phone number on the back of the flyer, as she is going to be at her grandma's house. Marty folds the flyer and puts it in his pocket. Pooh and his friends finally meet Marty and Pooh asks Marty if they could come with him to visit his house when goes there and Marty agrees.
When Marty gets home (along with Pooh and his friends, whom he invited over), he finds that the family car has been totaled by his father George's supervisor, Biff Tannen, who complains that George had not told him the car had a "blind spot". Biff then bullies George into writing his reports for work while making a rude comment by calling Marty a "butthead". Pooh and the others eventually meet George before dinner is ready. George decides to let Pooh and his friends join his family for dinner.
The family has dinner, during which it is discovered that Marty's older brother Dave works at a Burger King, his sister Linda has no love life, and his alcoholic mother Lorraine disapproves of girls chasing boys. She remarks that fate brought her and George together from her father hitting George with a car. Lorraine then mentions their first kiss at the "Enchantment Under the Sea" dance. Marty, Pooh, and the others fall asleep after dinner and is woken up by Doc calling him to bring Doc's video camera to the Twin Pines Mall. Marty, Pooh, and their friends meet Doc at the mall to witness and film a demonstration of Doc's latest invention: a time machine made from a modified DeLorean sports car, which must reach 88 miles-per-hour in order to travel through time. Doc also reunites with Pooh and the others, which made Marty ask Doc if he met them before and Doc says that it was a long story. Doc tests the car by sending his dog Einstein one minute into the future. Overjoyed by this success, Doc demonstrates to Marty how the time machine works by entering several significant dates into the keypad and telling him that the time machine needs 1.21 gigawatts of electrical power. Doc enters the date November 5, 1955 and explains to Marty that this was the day that he came up with the idea for the flux capacitor, the device that makes time travel possible.
After Doc refills the plutonium chamber of the time machine, a group of Libyan terrorists arrive, from whom he took the plutonium in exchange for a fake bomb. The Libyans shoot Doc, but Marty, Pooh, and their friends are able to escape in the DeLorean, accelerating to 88 miles per hour and thus inadvertently sending himself thirty years in the past to November 5, 1955.
Marty, Pooh, and the others, immediately, arrive at the Twin Pines Ranch, which was owned by Otis Peabody. Marty, Pooh, and their friends crash into the barn on Mr. Peabody's property which causes the entire Peabody family to wake up and come outside. On seeing Marty in his radiation suit and Piglet inside a pillowcase, it is concluded that Marty is an alien and Piglet is a ghost. Mr. Peabody then gets his shotgun to kill Marty, but Marty, Pooh, and the others are able to escape. Marty, Pooh, and their friends encounter many differences between 1985 and 1955, including a cleaner, less run-down ambiance in the Courthouse square. While searching for a younger Doc Brown, he and the others meet his father and accidentally interfere with the first meeting of his parents--being hit by his grandfather's Chevy in his father's place. (George had been in a tree watching Lorraine undress.) Lorraine falls in love with Marty instead, calling him "Calvin Klein" because she sees the name on his underwear. He (along with Pooh and his friends) has dinner with Lorraine's family, claims that he and the others have seen the episode of The Honeymooners ("The Man from Space") on TV even though it is brand new, and meets his Uncle Joey, a future "jailbird" who loves being in his playpen. Disturbed by his "mother's" flirtations, Marty, Pooh, and the others are shocked because she is entirely different from the prudish woman he remembers.
Marty, Pooh, and their friends quickly leave after feeling uncomfortable and find Doc, who disbelieves his story until Marty mentions the flux capacitor and how Doc came up with the idea. Doc tells Marty that his encounter with his parents jeopardizes Marty's own existence. A snapshot Marty carries of himself, Dave, and Linda documents this peril: part of Dave, the eldest, appears to have been "erased" from the photo, soon to be followed by Linda, and finally, unless disaster is averted, Marty.
After watching Marty's film of the time machine experiment, Doc is shocked to hear his future self explain that the DeLorean needs 1.21 gigawatts, and he runs off screaming "1.21 gigawatts?!" and Marty asks loudly "What the hell is a gigawatt?!" Doc realizes that he has no access to anything that would produce "1.21 gigawatts" of electrical power on demand and concludes that only harnessing the electrical power of a lightning bolt would give the flux capacitor that power it needs. Marty then shows him the flyer from 1985 that gives the exact time and place of a lightning bolt, one week away. Doc sets out to build a device that will let them channel the lightning bolt into the flux capacitor ultimately sending Marty, Pooh, and the others back to 1985. And Marty, Pooh, and their friends set out to put right, what they inadvertently made wrong between his parents.
Marty begins with trying to persuade George to ask Lorraine to the dance, but George is too nervous. They also have trouble with a younger Biff Tannen (the school bully), who is after Lorraine and forces George to do homework which he copies on the weekends. After a couple unsuccessful attempts at getting George and Lorraine together, while Pooh and the others stay with Doc, Marty dresses up in his radioactive suit and rudely awakes George while sleeping. Marty claims to be "Darth Vader" from the planet "Vulcan". The next morning, George finds Marty, Pooh, and their friends because he is now convinced that he must woo Lorraine. In Lou's Cafe, George attempts to woo Lorraine, but Biff comes in and harasses George. In an attempt to take up for his father, Marty trips Biff which leads to an exciting chase. Marty, Pooh, and the others run out of the cafe and creates a makeshift skateboard from a young boy's scooter. Marty, Pooh, and their friends are then able to outsmart and outmaneuver Biff and his gang, whom are in a car. Biff subsequently runs into a manure truck. The whole incident makes Lorraine even more attracted to Marty, and she asks Marty to the dance.
Marty accepts the invitation, but forms a plan for George to get Lorraine. Marty tells George that his plan is to 'take advantage' of Lorraine in the car, so that George can come rescue her. On the night of the dance, however, Lorraine is more than willing to let Marty take advantage of her, having swiped some liquor for the event, while Pooh and the others are in the car with Lorraine and Marty. (When Lorraine and Marty kiss, however, she is suddenly uncomfortable and says that it was like "kissing my brother".) Biff interrupts and gets in the car with Lorraine, while Biff's gang locks Marty in the trunk of singer-guitarist Marvin Berry's car. Pooh claims that Lorraine and Marty are in trouble and Piglet asks "Oh, what should we do, Pooh?". When George arrives, expecting Marty, he finds Biff harassing Lorraine instead. When Biff pushes Lorraine to the ground and laughs at her, George becomes infuriated, and knocks Biff out with a single punch to the chin. George and Lorraine head off to the dance just in time for Marty, Pooh, and their friends to see them together, having just been freed from Berry's car.
Nevertheless, the photograph is still fading and things have become worse. Berry sliced his hand open while trying to free Marty from the trunk, meaning that the live music is seemingly over, robbing George and Lorraine of the opportunity to kiss on the dance floor. Marty volunteers to play the guitar instead. During the first number, "Earth Angel", Mark Dixon, a student, cuts in between George and Lorraine. Dave and Linda have already disappeared from the photograph and Marty begins to fade out as well, both from the picture and in reality. George, exhibiting his new found confidence, pushes Dixon to the floor and proceeds to kiss Lorraine. At the moment of the kiss, Marty begins playing the guitar again with a renewed vigor and Dave and Linda have reappeared in the photograph, allowing Marty to know that the future is safe. At the band's request, Marty plays one more song, "Johnny B. Goode", which Pooh and the others are dancing to. Marvin Berry calls his cousin, Chuck Berry, 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".
Marty, Pooh, and their friends have a last chat with his parents and leaves to rejoin Doc Brown, who has suspended a cable from the top of the clock tower to channel the lightning into the DeLorean. Just as Marty, Pooh, and the others are getting ready to take the DeLorean to the starting line Doc drew, Doc discovers a letter that Marty had written earlier describing Doc's death. Doc, not wanting to endanger his future by knowing too much, tears it up unread. A tree limb falls onto the cable, disconnecting it. Doc puts the torn pieces of the letter in his coat pocket and runs to fix the cable. Marty continually tries to tell Doc about the future, but runs out of time. Marty (along with Pooh and his friends), in frustration, gets into the DeLorean, and sets its time circuits to take him back earlier than he left, so he can save Doc. Meanwhile, Doc fixes one wire only to disconnect another. He slides down the wire and reconnects it, just as the lightning hits the tower. Marty, Pooh, and the others accelerate to 88 miles per hour and contacts the cable just as the lightning speeds through the electrified wire, sending the DeLorean back to the future.
Marty, Pooh, and their friends return to 1985 11 minutes before he left, but the car stalls again. While frantically trying to start the car, the Libyans drive past and Marty, Pooh, and the others have no choice but to run to the mall. Marty, Pooh, and their friends arrive too late, and see Doc being shot and themselves driving the DeLorean and going back to 1955 while the terrorists crash into a photo booth. Marty, Pooh, and the others rush down to Doc's body and turn away in tears. Doc suddenly sits up, opening his radiation suit to reveal a bulletproof vest. He then pulls out the letter Marty wrote him, yellowed and taped back together from the shreds he tore it into 30 years before. Doc drives Marty home, and then departs for the year 2015 before taking Pooh and his friends back to the Hundred Acre Wood. In the morning, Marty discovers his house is different; there is a new BMW in the driveway (in place of the wrecked Nova), Linda has an active social life and Dave has an office job. Lorraine and George arrive home from playing tennis, both more fit and attractive, and much more affectionate to one another than when Marty left. George also shows more self confidence, even catching Biff in a fib. Biff, who instead of being George's supervisor now has an auto detailing service, runs in with the delivery of George's first novel, a science fiction story called A Match Made In Space. Marty then finds that the Toyota pick-up truck that he previously coveted is now his. As Marty and Jennifer are about to take a ride in the truck, Doc (along with Pooh and the others with a little surprise) reappears in the new and improved DeLorean, telling Marty to come back to the future with him, and says that something has got to be done about their kids (this will be explained fully in Part II). Before he hurries Marty and Jennifer into the car, Pooh and his friends reunite with Marty, with Pooh telling him that they have a surprise for him. The surprise is another gang Doc also invited over: Kanga, Roo, Lumpy, Darby, Buster, Ash Ketchum, Pikachu, Misty, Brock, Togepi, Tennessee Tuxedo, and Chumley. As Ash, Tennessee, and their friends automatically meet Marty and Jennifer, Ash says that they should all go into the DeLorean. As Doc pulls out of the driveway, Marty points out that there is not enough road to accelerate to 88 mph, with Doc replying, "Roads? Where we're going, we don't need roads," and flies off in the now fusion-powered and hover-converted car. As it ends, Genie stops the film and says "Made you look" and puts it back with the slide saying "To be continued in Winnie the Pooh Goes Back to the Future Part II".
- Ash Ketchum, Pikachu, Misty, Brock, Togepi, Kanga, Roo, Lumpy, Darby, Buster, Tennessee Tuxedo, and Chumley will appear at the end of the movie.
- Genie also appears at the end of the film just by stopping it and saying "Made you look".
- It is revealed that Pooh and his friends had met Doc before.
- This is an NTSC film with NTSC bits from the Winnie the Pooh movies and the Pokemon movies.