[Marty sees the outside of the Hill Valley High School in 1955]Marty McFly: Whoa. They really cleaned this place up. Looks brand-new.[Marty and Doc walk toward the building]Dr. Emmett Brown: Now, remember. According to my theory, you interfered with your parents first meeting. If they dont meet, they wont fall in love, they wont get married and they wont have kids. Thats why your older brothers disappearing from that photograph. Your sister will follow, and unless you repair the damage, youll be next.Marty McFly: Sounds pretty heavy.Dr. Emmett Brown: Weight has nothing to do with it.

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