Zemeckis and Gale wrote the script after Gale mused upon whether he would have befriended his father if they attended school together. Various film studios rejected the script until the financial success of Zemeckis' Romancing the Stone. Zemeckis approached Spielberg and the project was planned to be financed and released through Universal Pictures. Eric Stoltz was originally cast as Marty McFly when Michael J. Fox was busy filming the TV series Family Ties. However, during filming, Stoltz and the filmmakers decided that he was miscast, so Fox was approached again and he managed to work out a timetable in which he could give enough time and commitment to both.

Back to the Future was released on July 3, 1985 and became the most successful film of the year, grossing more than $383 million worldwide and receiving critical acclaim. It won the Hugo Award for Best Dramatic Presentation and the Saturn Award for Best Science Fiction Film, as well as an Academy Award, and Golden Globe nominations among others. Ronald Reagan even quoted the film in his 1986 State of the Union Address. In 2007, the Library of Congress selected it for preservation in the National Film Registry, and in June 2008 the American Film Institute's special AFI's 10 Top 10 designated the film as the 10th-best film in the science fiction genre. The film marked the beginning of a franchise, with sequels Back to the Future Parts II and III released in 1989 and 1990, as well as an animated series, theme park ride and several video games.

Plot
Seventeen-year-old Marty McFly lives with his bleak, unambitious family in Hill Valley, California. His father, George McFly, is bullied by his supervisor, Biff Tannen, while his unhappy mother, Lorraine Baines McFly, is an alcoholic. Marty's underachieving older siblings, Dave and Linda, also live in the household. When Marty and his band audition to perform at the high school dance, they are rejected. Despite this setback, Marty's girlfriend, Jennifer, encourages him to pursue the dream of being a rock musician. At dinner that night, Lorraine recounts how she and George first fell in love when her father hit George with his car.

Marty meets his friend, scientist Dr. Emmett "Doc" Brown, late at night in the parking lot of a deserted shopping mall where Doc reveals a time machine made from a modified 1981 DeLorean DMC-12; the vehicle's time displacement is powered by plutonium, which supplies the 1.21 gigawatts of power to a device he calls the "flux capacitor." Doc explains that the car travels to a programmed date upon reaching 88 miles per hour, using the date November 5, 1955, as an example destination. Before Doc can make his first trip, the Libyan terrorists from whom he stole the plutonium shoot him. Marty attempts to escape in the DeLorean and inadvertently activates the time machine. He is transported back to November 5, 1955, and finds himself without the plutonium needed for the return trip.

While exploring the 1955 version of Hill Valley, Marty meets his teenaged father who is being bullied by Biff and is writing unshared science fiction stories. As George is about to be hit by Lorraine's father's car, Marty pushes him out of the way and is knocked out by the impact. Consequently, a teenaged Lorraine becomes infatuated with Marty instead of George. Marty is disturbed by her flirtations and leaves to find the younger Doc of 1955. Marty convinces Doc that he is from the future, and asks for help returning to 1985. Doc explains that the only available power source capable of generating 1.21 gigawatts of energy is a bolt of lightning. Discovering the "Save the Clock Tower" flyer that Marty received in 1985, indicating that lightning will strike the courthouse clock tower the following Saturday at 10:04 pm, Doc makes plans to harness the lightning strike to power the DeLorean's flux capacitor. When they observe a fading photograph of Marty with his siblings, they realize Marty has prevented his parents from meeting, jeopardizing his family's existence.

Marty attempts to set George up with Lorraine. To make his parents fall in love, Marty plans to have George "rescue" Lorraine from Marty's inappropriate advances on the night of the school dance. A drunk Biff unexpectedly shows up, pulls Marty from the car, and attempts to force himself on Lorraine. George arrives to rescue her from Marty, but instead finds Biff, who humiliates George and pushes Lorraine to the ground. Standing up to him for the first time, George knocks Biff out. A smitten Lorraine follows George to the dance floor, where they kiss for the first time, ensuring Marty's existence.

Marty arrives at the clock tower where Doc is making final preparations for the lightning strike, and tries to warn Doc of his impending 1985 murder in a letter, but Doc tears it up, fearing it will lead to altering the future. A falling tree branch disconnects Doc's wiring setup, but Doc repairs the connections in time to send Marty and the DeLorean back to 1985. Although Marty arrives too late to prevent him from being shot, Doc is still alive and admits to reading the letter anyway and wearing a bulletproof vest.

Doc drops Marty off at home and uses the time machine to travel 30 years into the future. Marty awakens the next morning to find his family changed; Lorraine is happy and physically fit, a self-confident George is a successful science fiction author, Dave is an office employee, and Linda no longer has trouble finding boyfriends. George and Lorraine now have a closer relationship than ever, while Biff has become an auto detailer/washer who is on good terms with the McFly family. As Marty reunites with Jennifer, Doc arrives, insisting they accompany him to the future to sort out a problem with their future children. Marty and Jennifer enter the upgraded DeLorean, now a hovercar powered by nuclear fusion, and Doc flies the time machine into the future.

Critical response
Back to the Future received universal acclaim. At Metacritic, which assigns a normalized rating out of 100, the film received an average score of 86, which indicates "universal acclaim", based on 12 reviews.  As of September 2012, review aggregator Rotten Tomatoes reported that 97% of critics gave the film a positive review, based on 61 reviews, certifying it "Fresh", with an average rating of 8.6 out of 10 and the consensus: "Inventive, funny, and breathlessly constructed, Back to the Future is a rousing time-travel adventure with an unforgettable spirit."

Roger Ebert felt Back to the Future had similar themes to the films of Frank Capra, especially It's a Wonderful Life. Ebert commented " Steven Spielberg is emulating the great authentic past of Classical Hollywood cinema, who specialized in matching the right director (Robert Zemeckis) with the right project." Janet Maslin of The New York Times believed the film had a balanced storyline: "It's a cinematic inventing of humor and whimsical tall tales for a long time to come." Christopher Null, who first saw the film as a teenager, called it "a quintessential 1980s flick that combines science fiction, action, comedy, and romance all into a perfect little package that kids and adults will both devour." Dave Kehr of Chicago Reader felt Gale and Zemeckis wrote a script that perfectly balanced science fiction, seriousness and humor. Variety applauded the performances, arguing Fox and Lloyd imbued Marty and Doc Brown's friendship with a quality reminiscent of King Arthur and Merlin. BBC News applauded the intricacies of the "outstandingly executed" script, remarking that "nobody says anything that doesn't become important to the plot later." Back to the Future appeared on Gene Siskel's top ten film list of 1985.

Awards
At the 58th Academy Awards, Back to the Future won for Best Sound Editing while "The Power of Love" was nominated for Best Song. Bill Varney, B. Tennyson Sebastian II, Robert Thirlwell and William B. Kaplan were nominated for Best Sound Mixing, and Zemeckis and Gale were nominated for Best Original Screenplay. The film won the Hugo Award for Best Dramatic Presentation and the Saturn Award for Best Science Fiction Film. Michael J. Fox and the visual effects designers won categories at the Saturn Awards. Zemeckis, composer Alan Silvestri, the costume design and supporting actors Christopher Lloyd, Lea Thompson, Crispin Glover and Thomas F. Wilson were also nominated. The film was nominated for numerous BAFTAs at the 39th British Academy Film Awards, including Best Film, original screenplay, visual effects, production design and editing. At the 43rd Golden Globe Awards, Back to the Future was nominated for Best Motion Picture (Musical or Comedy), original song (for "The Power of Love"), Best Actor in a Motion Picture Musical or Comedy (Fox) and Best Screenplay for Zemeckis and Gale.