When Doc takes Marty to the year 2015 to prevent his future son from committing a crime, old Biff takes the Gray's Sports Almanac
and travels back to 1955 and gives the book to his younger self. Over
the next three decades, young Biff uses the information to win money on
the outcome of each sporting event he bets on, making him the richest
and most powerful man in the world by 1985A.
To ensure that No one else gets their hands on the almanac, Biff
travels through time and collects various items and puts them behind
locked doors in three different periods. This action has caused a
disruption in time, and now the Space Time Continuum is falling apart. With Hill Valley
in ruins, Marty must now travel to all three periods, then find and
return all the items back to their original places in order to retrieve
the almanac and destroy it.
After Marty returns all items to their rightful places and burns the almanac, lighting strikes the DeLorean
and sends Doc back in time, leaving Marty stranded in 1955 once again.
At the same time, Marty receives a letter from Doc via a courier that
was written in 1888. According to the old letter, Doc has become
stranded in the year 1888 and must be rescued or else this will start
another time-ripple effect. Now, with the help from 1955 Doc, Marty
must go back to 1888 in order to rescue the present-day Doc with the
(aged but repaired) DeLorean. Before they can return back to 1985,
Marty must find and return ten more items to their rightful location.
To impede his progress, outlaw Buford Tannen has other plans.