On November 12, 1955, Marty McFly, before returning to 1985, wrote Doc Brown a letter on letterhead from Lou's Cafe, using a fountain pen, and sealed in an envelope marked "DO NOT OPEN UNTIL 1985". It alerted Doc to the Libyan terrorists that would eventually shoot him back at Twin Pines Mall on October 26, 1985.
Marty placed the letter in Doc's coat pocket. However, when Doc discovered the letter, he tore it up and placed the remains back into his pocket. Sometime in the thirty years before the event, Doc taped it back together and read it. Following its instructions, he wore a bulletproof vest on the night of the world's first temporal displacement. When Marty arrived back in 1985 and discovered that his friend was still alive, Doc produced the letter – now yellow with age and the tape holding it together ready to fall apart – from his inside pocket.
"Dear Dr. Brown
On the night that I go back in time at 1:30 AM, you will be shot by terrorists.
PLEASE take whatever precautions are neccessary to prevent this terrible disaster.
Behind the scenesBob Gale was surprised to get a letter from a young fan in Japan who pointed out that Marty's letter looked different in 1985 than it had in 1955. "Some people don't have enough to do, I guess," commented Gale. Basically, the letter in the first scene concludes with four lines ("Please take whatever/ precautions are necessary/ to prevent this terrible/ disaster"); when Doc taped it back together, the same sentence was on three lines, as seen in the image above. Like the DeLorean odometer, the McFly family candy jar, and the flaps on the shirt pockets, the letter is one of the continuity errors that fans of the films enjoy spotting.
After 2001, in light of the 9/11 terrorist attack on the Twin Towers in New York, whenever Back to the Future was shown on television, the scene in which Marty is reading his letter was sometimes edited so that he says, "Dear Dr. Brown, on the night I go back in time you'll be shot. Please, please take whatever..." (The words "by terrorists" were redubbed with "please" and were digitally erased from the letter when it was shown on-screen in close-up.)