The story opens in Dr. Emmett Brown's (Christopher Lloyd) garage/home laboratory as the camera pan over a large collection of clocks. A robotic tin can opener opens a tin of spoiled dog food and empties the contents into an overflowing dog food bowl marked "Einstein". The television set and radio turn on. On the TV, we see the ending of an advertisement, followed by a woman newscaster announcing the recent theft of a case of plutonium.
The front door of the garage opens, and Marty McFly (Michael J. Fox) comes in. Marty calls out, then reaches down to partly lift up the doormat. He places a set of keys under the doormat, then drops it back down. Marty enters the garage, calling out for Doc and whistling for Einstein. He comments on the mess the place is in. Marty puts down his skateboard and it rolls along the floor until it hits a box of plutonium.
He turns on Doc's amplifier system, turning all the settings to maximum. A hum grows louder in the background. Marty plugs his electric guitar into a huge amplifier, pauses, and then plucks a string. The amplifier blows up, the impact throwing Marty back against a bookshelf, which falls, causing the books and papers on it to fall off and land on his head. Marty lifts up his sunglasses and we finally get to see his whole face. "Whoa... rock and roll," he says, when a loud ringing fills the garage. It sounds like a fire alarm, but then turns out to be just the telephone.
Marty scrambles off the ground and answers it. It's Doc, who asks Marty to meet him that night at the Twin Pines Mall at 1:15 a.m.. Marty asks him where he's been all week. Doc says that he has been working. Marty tells him that his equipment had been left on all week. Remembering, Doc tells Marty not to hook up to the amplifier. "There's a slight possibility of overload," he says. Marty glances at the destroyed amplifier, and says that he'll keep that in mind. Just then, every single one of the numerous clocks go off at once, chiming loudly, and Doc asks about them.
Marty tells him that it's eight o'clock. Doc is elated at the information, as it means that his experiment has worked and all his clocks are 25 minutes slow....meaning it really is 8:25, and Marty is late for school. He exclaims this news into the telephone, slams down the receiver, retrieves his skateboard and rushes out of the garage. Marty gets on his skateboard and skates through the streets, hitching a ride first on a pickup truck and then on another Jeep to get through town. Marty arrives outside Hill Valley High School. He hops off his skateboard and flips it up into his hand.
His girlfriend Jennifer Parker (Claudia Wells) is waiting for him. She warns him that the principal, Mr. Strickland (James Tolkan) is looking for him. Marty tells her that his lateness is not his fault, because Doc set his clocks slow. Strickland suddenly appears at the sound of Doc's name. He demands to know if Marty is still hanging around with Doc, and hands him and Jennifer a tardy slip each; it is Marty's fourth in a row.
Strickland warns Marty that Doc is a dangerous nuctase, and if he continues hanging out with him, he'll get in trouble. Strickland also harshly tells Marty that he is a slacker, just like his father. "No McFly ever amounted to anything in the history of Hill Valley," he says, bringing his face closer to Marty's until their noses touch. Marty counters that history will be changing soon.
We change to the auditorium, where a band has just finished playing. Four judges sit on chairs before the stage and request the next band. Marty and his band get up on stage and he introduces them as The Pinheads before launching into the opening bars of The Power of Love. One of the judges (the song's artist, Huey Lewis, in a cameo appearance) cuts them off and tells them that they are too loud.
After school, Marty and Jennifer are walking through the Courthouse Square as a mayoral campaign van drives past, blaring "Re-elect Mayor Goldie Wilson!" over its loudspeakers. Marty tells Jennifer about how he doubts he'll ever get anywhere with his music. Jennifer tries to reassure him with her opinion that he's really good, and encourages him to send in his audition tape to the record company, but Marty expresses fear that they'll reject him.
He looks up as a new 4x4 Toyota pickup truck is delivered to the Statler Toyota dealership across the street, and admires it, musing about taking Jennifer in it for a weekend trip to the lake. Jennifer asks if Marty's mother knows about their plans for the next night. Marty assures her that his mother thinks he's going camping with the guys, and that she would freak out if she knew the truth. Marty fears his mother was probably born a nun. Jennifer assures him that she's just trying to keep him respectable.
Their lips come closer, but just as they are about to kiss, a tin can is shoved in their faces by a woman shouting "Save the clocktower! Save the clocktower!" The woman asks them to deposit money that will be contributed to a fund to save the clock tower, which has been frozen at 10:04 ever since it was struck by lightning at that exact time on the night of November 12, 1955. The mayor would like the clock to be replaced, the Hill Valley Preservation Society thinks that it is important part of their heritage and should be left alone. Marty gives her a quarter just to get her to go away.