In 1985, Hill Valley, California, Marty McFly visits the home and lab of his friend, Emmett “Doc” Brown, an eccentric scientist. Though missing for several days, Doc calls the lab and asks Marty to pick up some equipment for a special experiment later that night.
At school, Marty meets his girlfriend, Jennifer Parker, and after being berated by the principal and failing his audition for the Battle of the Bands, he confides in Jennifer that he fears that he will become like his parents despite his ambitions. At home, Marty’s cowardly father George is bullied by his supervisor, Biff Tannen, while his mother Lorraine is an overweight, depressed alcoholic. Lorraine recalls how she met George when her father hit him with his car and she subsequently nursed him back to health.
Marty meets Doc at a parking lot by a shopping mall at 1:16 A.M. on October 26. Doc unveils a time machine built from a modified DeLorean and powered by plutonium obtained from Libyan terrorists with the promise of building a nuclear bomb. While showing Marty the controls, Doc sets the date to November 5, 1955, the day he conceived a time travel device. The terrorists arrive unexpectedly and open fire on them, finally shooting Doc when he surrenders. Marty escapes in the DeLorean and inadvertently activates the time machine in the process.
Marty finds himself transported to November 5, 1955, without any plutonium to return. He encounters the teenage George and discovers that Biff has been bullying him since high school. After Marty saves George from an oncoming car, he is knocked unconscious and awakens to find himself tended to by Lorraine, who becomes infatuated with him.
Marty tracks down Doc’s younger self for help. Doc is unimpressed with Marty’s knowledge of the future but finally believes him when he accurately recounts how Doc invented time travel. With no plutonium, Doc explains that the only power source capable of generating the necessary 1.21 gigawatts (1,620,000 hp) of electricity for the time machine is a bolt of lightning. Marty shows Doc a flyer from the future that recounts a lightning strike at the town’s courthouse due the coming Saturday night, November 12. When Doc instructs Marty to not leave his house or interact with anyone, as he could inadvertently alter the future, they realize that he has prevented his parents from meeting by saving George from the car. Doc warns Marty that he must find a way for Lorraine to fall in love with George or he will be erased from existence. Marty attempts to correct his parents’ history and warn Doc of the future dangers, but both strategies fail: his attempts to introduce his parents backfire during a confrontation with Biff and Doc refuses to listen to any information about the future, saying no one should know too much about their own destiny.
After Lorraine asks Marty to the school dance, Marty devises a plan with George: he will feign inappropriate advances on Lorraine, allowing George to “rescue” her. The plan goes awry when a drunken Biff gets rid of Marty, making his gang lock him in the trunk of the performing band’s car and attempts to force himself on Lorraine. Marty is then rescued by the band who unlock the trunk and drive away Biff’s gang and rushes to stop him. Meanwhile, George, enraged, knocks out Biff, and Lorraine accompanies him to the dance floor, where they kiss while Marty performs with the band, securing his existence in the future.
As the storm arrives, Marty returns to the clock tower and the lightning strikes, sending Marty back to 1985 a few minutes before his original departure. After watching Doc get shot again, he grieves until Doc gets up and reveals that he had listened to Marty’s warnings and worn a bulletproof vest. Doc takes Marty home and departs to the future. Marty awakens the next morning to find that his father is now a self-confident and successful author, his mother is fit and happy, and Biff is now an obsequious auto valet. As Marty reunites with Jennifer, Doc reappears with the DeLorean and insists that they return with him to aid their future children. The trio boards the DeLorean which now flies and travels into the future.