Doc quotes from Back to the Future Part II (1989)

They're taking her home, to your future home! We'll arrive shortly thereafter, get her out of there, and go back to 1985.

Marty

You mean, I'm gonna see where I live? I'm gonna see myself as an old man?

No, no, no, Marty. That could result in a [gasps] Great Scott! Jennifer could conceivably encounter her future self! The consequences of that could be disastrous!

Marty

Doc, what do you mean?

I foresee two possibilities. 1: coming face to face with herself 30 years older would put her into shock and she'd simply pass out. Or 2: the encounter could create a time paradox, the result of which could cause a chain reaction that would unravel the very fabric of the spacetime continuum and destroy the entire universe! Granted, that's a worst-case scenario. The destruction might in fact be very localised, limited to merely our own galaxy.

Marty

Well, that's a relief.

Marty

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.

Marty

I'll ask him.

Marty

Doc, Doc! [grabs Doc, who shouts in shock] Okay. Okay! Relax, Doc! It's me! It's Marty!

No! It can't be! I just sent you back to the future.

Marty

I know. You did send me back to the future, but I'm back. I'm back from the future.

Great Scott! [faints]

Obviously, the time continuum has been disrupted, creating this new...temporal event sequence resulting in this alternate reality.

Marty

English, Doc.

Here, here, here. Let me, uh, let me illustrate. [brings over a chalkboard] Imagine that this line [draws a straight line across the board] represents time. Here's the present, 1985, the future and the past. [writes "Past" on left side, "1985" in the middle, and "F" for Future on the right; points at the area before 1985] Prior to this point in time, somewhere in the past, the timeline [draws another line] skewed into this tangent creating an alternate 1985. [writes 1985A on chalkboard] Alternate to you, me and Einstein, but reality for everyone else. [pulls a small, silver plastic bag out of the DeLorean] Recognize this? It's the bag the sports book came in. I know, because the receipt was still inside. I found them in the time machine...along with this. [shows Marty a small-fisted object]

Marty

It's the top of Biff's cane. I mean, Old Biff from the future.

Correct. It was in the time machine because Biff was in the time machine, with the Sports Almanac!

Marty

Holy shit.

You see, while we were in the future, Biff got the sports book, stole the time machine, [draws an overarching line across the chalkboard] went back in time, and gave the book to himself at some point in the past. [brings over a large newspaper] Look. [scoffs] It says right here that Biff made his first million betting on a horse race in 1958. He wasn't just lucky. He knew because he had all the race results in the Sports Almanac. That's how he made his entire fortune! Look at his pocket with a magnifying glass.

Marty

The Almanac. [angrily] Son-of-a-bitch stole my idea! He must have been listening when I-- [realizes; shocked] It's my fault. The whole thing is my fault. If I hadn't bought that damn book, none of this would have ever happened.

That's all in the past.

Marty

You mean the future.

Whatever! It demonstrates precisely how time travel can be misused, and why the time machine must be destroyed, after we straighten all of this out.

Marty

Right. 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... [indicates 1985A on the blackboard] reality, in which Biff is corrupt, and powerful, and married to your mother, and in which... [holds up a newspaper; the headline reads: "EMMETT BROWN COMMITTED"] ...this has happened to me! [Einstein moans, and Marty takes the newspaper from Doc] 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, where, and when young Biff got his hands on that Sports Almanac.

Marty

I'll ask him.