The second part of the trilogy begins as Doc Marty and Jennifer take the time-travelling DeLorean into the year 2015 to straighten out the future of the McFly family. But Biff Tannen steals the time machine and gives his younger self a book containing 50 years of sports statistics which the young Biff uses to amass an enormous gambling fortune and transform idyllic Hill Valley into a living hell. To restore the present Doc and Marty must return to the events of their previous adventure in 1955 and retrieve the book.