Deliberate mistake: When Old Biff and Young Biff are sitting in Biff's car, the car's rear-view mirror vanishes during all shots of Old and Young Biffs together. Presumably this made the split-screen effect easier to do.
Dr. Emmett Brown takes Marty and Jennifer to the future to help their kids not to ruin their lives.
After doing that, Marty buys a sports almanac for 1950-2000, thinking it might help him in his time. Doc finds out and throws it away, but the 2015's Biff (very old now) finds it and uses the DeLorean to give the almanac to the young himself.
When Marty and Jennifer arrive back to their own time, they find it totally different - Biff is very rich, he practically owns the city, he married Marty's mom, killed his dad, and when Marty starts to ask about the almanac, he wants to kill him too.
Doc also finds out that he was imprisoned as a lunatic, so the two go back to 1955, the night of the Enchantment-Under-The-Sea party to take back the almanac.
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