OK old biff finds the Almanac (book of sporting facts and finds out the Doc Built a time machine .he steels the time machine while it is parked up at the lyons Estate in the future, he goes back in time to 1955 and gives the book to himself in high school,then comes forward in time and places the delorean back in the same place he stole it from in the 1st place (while doc and marty are getting jennifer) then old biff evaporates because of the changes in the time line. OK problem now is when marty and doc go back to 1985 they find that everything has changed because of old biffs interfearence.marty says they should go forward in time and stop old biff from getting the almanac in the 1st place ,,Doc tells us that they cannot because the future they whould go to whould be the future where biff was rich and not the future to where they where origninaly supposed to live in... now if this is true how did old biff make it back to the future to re-deliver the DMC .. is this a mistake in docs ideas or is it a mistake in the film ???now if you argue that old biff returned before he used the book then simply all marty and doc whould need to do is go back to 1955 and then go forward about the same time old biff did,,dont say they couldnt have figured this out doc made a time machine for heks sake.