In 1985 Hill Valley, California, teenager Marty McFly and his girlfriend, Jennifer Parker, are chastised by the school principal for lateness. Marty auditions for the Battle of the Bands, but is rejected for being too loud. At home, Marty's 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.

Marty is invited by his friend, eccentric inventor Dr. Emmett Brown, to meet him in a parking lot in the early hours. Doc unveils a time machine built from a modified DeLorean and powered by plutonium stolen from terrorists. Preparing to demonstrate the time machine, Doc sets the date to November 5, 1955: the day he invented time travel. The terrorists arrive and shoot Doc. Marty escapes in the DeLorean, but inadvertently activates the time machine.

Marty finds himself in 1955 without enough plutonium to return. He encounters the teenaged George, who is bullied by his classmate Biff. After Marty saves George from an oncoming car, he is knocked unconscious and awakens to find himself tended to by Lorraine, who is infatuated with him.

Marty tracks down Doc's younger self for help. With no plutonium, Doc explains that the only power source capable of generating the necessary 1.21 gigawatts 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. Doc instructs Marty to not leave his house or interact with anyone, as he could inadvertently alter the future; because of this, Doc refuses to heed warnings from Marty about his death in 1985. When they realize that he has prevented his parents from meeting, Doc warns Marty that he must find a way to introduce George to Lorraine or he will be erased from existence. Doc formulates a plan to harness the power of the lightning, while Marty sets about introducing his parents.

After Lorraine asks Marty to the school dance, Marty concocts a plan: he will feign inappropriate advances on Lorraine, allowing George to "rescue" her. The plan goes awry when a drunken Biff attempts to force himself on Lorraine. George, enraged, knocks out Biff, and Lorraine follows him to the dance floor, where they kiss while Marty performs with the band.

As the storm arrives, Marty returns to the clock tower and the lightning strikes, sending Marty back to 1985. Doc has survived the shooting, as he had listened to Marty's warnings and worn a bullet-proof vest. Doc takes Marty home and departs to the future. Marty awakens the next morning to find that George is a successful author, Lorraine is fit and happy, and Biff is now an obsequious auto valet. As Marty reunites with Jennifer, the DeLorean appears with Doc, insisting they accompany him to 2015 to resolve a problem with their future children. The trio board the DeLorean, which has been upgraded with hover technology, and warp to the future.