Hmm, that reminds me. In the first film, the conflict is really played up as Marty's problem. If Marty doesn't fix it, then Marty will cease to exist. It isn't until the second film that they begin referring to time paradoxes that can "unravel the very fabric of the space time continuum and destroy the entire universe." But if you think about it, the conflict in the first film would have been that as well. If Marty ceased to exist by interfering with his parents getting together, he would not have existed to interfere with his parents getting together. Boom. Hmm, maybe Doc should have taken this a bit more seriously. =P