Marty: "I don't get it. It's like we're in Hell or something!"
Doc: "No, it's Hill Valley, although I can't imagine Hell being much worse! I'm sorry, Einie, the lab is an awful, awful, awful mess!"
Doc (continuing): "Obviously the timeline has been disrupted, creating a new temporal event sequence, resulting in this Alternate Reality."
Marty: "English Doc!"
Doc (writing on a chalk board): "Here, here, here! Let me illustrate. Imagine that this line represents time: the future, the present, the past. Part of this point in time, somewhere in the past, the timeline skewed into a tangent, creating an Alternate 1985. Alternate to you, me, and Einstein, but reality for everyone else."
Doc (pulling out something): "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!"
Marty: "It's the top of Biff's cane. I mean Old Biff, from the future!"
Doc: "Correct! It was in the time machine because Biff was in the time machine, with the Sports Almanac! You see, while we were in the future, Biff got the sports book, stole the time machine, went back in time and gave the book to himself at some point in the past."
Doc (reaching for Newspapers): "Look! It says right here that Biff made his first million dollar 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 the magnifying glass."
Marty: "The Almanac. Son of a bitch! Stole my idea! He must've been listening when... It's my fault. The whole thing is my fault! If I hadn't bought that damn book, none of this would've ever happened!"
Doc: "It's all in the past."
Marty: "You mean the future."
Doc: "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!"
Doc: "We can't because if we travel into the future from this point in time it will be future of THIS reality! A reality in which Biff is corrupt, and powerful, and married to your mother, and in which this has happened to me!" (Doc holds up a newspaper that says he's been committed)
Doc (continuing): "No. The only way to repair the present is in the past, at the point in which the timeline skewed into this tangent! In order put the universe back as we remember it and get back to our reality, we have to figure out the exact date and the specific circumstances of how, where, and when Young Biff got his hands on that Sports Almanac."
Marty: "I'll ask him."