biff tannen steal's doc's delorean time machine he return to his past in 1955 to restore the timeline, but biff killed marty's father in march 15 1973 and he forced to married marty's mother, biff tries to kill marty he travels back to novmeber 12 1955, the lightning strikes hits in november 12 1955, to a 70 year old letter it's from the doc he transported to september 1 1885, marty races back into town to find 1955 marty is returned to 1985.