So we go back to the future, and we stop Biff from stealing the time machine.

We can't, because if we travel into the future from this point in time, it will be the future of this reality [indicates the alternate reality on a blackboard, 1985A], in which Biff is corrupt, powerful, and married to your mother, and in which this has happened to me! [holds up a newspaper; the headline reads: "EMMETT BROWN COMMITTED"] No. Our only chance to repair the present is in the past, at the point where the timeline skewed into this tangent. In order to put the universe back as we remember it and get back to our reality, we have to find out the exact date and specific circumstances of how, when, and where young Biff got his hands on that sports almanac.

I'll ask him.