The major plot
hole in this series is in the second movie, which does, in its own way, deal with alternate
dimensions or realities
. Following along with Doc
's explanation of what old Biff did, we have one timeline
(or dimension) that is the standard (Earth-Prime, as it's called in "Sliders
."). We are living happily in that timeline, traveling back and forth within it, trying not to change things because that would inevitably corrupt
said timeline and possibly end existence
, as Doc puts it.
So, one day in 2015, old Biff gets fed up with his life, steals the time machine and the almanac and goes back in time to 1955 to visit his former self and improve his life by making himself rich and famous. By doing this, he creates a new timeline, which, up to the point where he entered and changed it, was identical to the original timeline ("Prime"), thus allowing for Doc and Marty to go back to a time before all this happened in order to fix things.
The problem is this: Old Biff could not have traveled back to the original timeline with the DeLorean.
In the first movie, when Marty returns to 1985, he finds everything changed positively, while he is unaware of the events in the new timeline. He changed events in 1955, so when he traveled forward, he traveled forward into the new timeline he created, rather than the old one he came from in which Biff was still a bully and his family was generally dysfunctional. Therefore, the same situation would have occurred with old Biff. When he returned to 2015, it would have been the altered 2015, in which he was rich and powerful. Instead, he returns to the original timeline, where Marty and Doc are visiting a grown-up Marty and rescuing Jennifer. Following the premise laid out in the first movie, Biff wouldn't have been able to return to that reality, and therefore Doc and Marty wouldn't have been able to fix the situation, as they would have been altered along with everything else in the timeline.
The only possible explanation for this is that it takes a certain amount of "time" for the timeline change to take effect. In the first movie, Marty went to sleep shortly after he returned to 1985, so the time change may not have taken place yet, but by the time morning came, everything had been altered with the new timeline (The so-called "ripple" or "wave" effect).
This being the case, old Biff returned to the old timeline before it had changed, Doc and Marty got in the time machine and returned to 1985, where the timeline-change had already taken effect. Had they returned to 2015, it would have been an altered 2015. This seems to make sense, but there are two distinct problems:
1) Following this logic, Marty in the first movie would have become, in his sleep, the new Marty (the Marty who existed as a result of his time-traveling and the timeline-altering events he caused), which would have meant his "new" surroundings (nice house, parents who get along, lackey Biff) would have been familiar to him. Instead, he was terribly confused.
2) This theory requires that there is another type of time, one that effects time over time. If this didn't exist, any change you made in the timeline would take effect immediately, and the time traveler would find himself with new memories that had somehow always been there (this is dealt with surprisingly well in the cartoon "Invader Zim."). But, since this is clearly not the case in “Back To The Future”, there must be some kind of ripple that starts at the place (time) of origin of the alteration and slowly moves forward, changing time to fit the new timeline. Under this logic, making a trip through time would have to take the amount of time that the actual journey takes, rather than being instantaneous, as it is clearly explained in the movies. So, even if we assume this theory is possible, as ridiculous as it seems, and disregard problem number 2, it still follows that Marty would have to be really stupid and out of it to not notice the way his family has always been (according to the altered timeline he created).
Either way, there’s a problem. But, they are still great movies. Otherwise, they wouldn’t even be worthy of this much discussion.