The day after Marty McFly (Michael J. Fox) returns from 1955, Dr. Emmett Brown (Christopher Lloyd) shows up at the McFly house to pick up Marty and intervene with an event that may destroy Marty's future family. Along with Marty's girlfriend Jennifer (Elizabeth Shue), Doc and Marty head thirty years into the future. Doc has Marty take his future sons place and turn down an offer from Biff's grandson, Griff (Thomas F. Wilson) that could lead to potential trouble. Once Marty is finished with business, he picks up a "souvenir" that could give him an advantage back in his own time period. Doc finds the item and throws it out, but unfortunately Biff finds the discarded item and takes the Delorian back to 1955 to give the item to his younger self. Marty and Doc don't realize that this has happened until they return to 1985 and see that it is nothing like when they left.
So, Doc and Marty must go back to 1955 and retrieve the item that old Biff gave to his younger self. If they are unsuccessful they will be stuck in the "new" 1985. Now back in 1955 there are two Doc Browns and two Marty McFlys. Both men need to try and avoid the other versions of themselves while trying to repair the damage caused and make sure their futures get back on track to what they are supposed to be.
So, Doc and Marty must go back to 1955 and retrieve the item that old Biff gave to his younger self. If they are unsuccessful they will be stuck in the "new" 1985. Now back in 1955 there are two Doc Browns and two Marty McFlys. Both men need to try and avoid the other versions of themselves while trying to repair the damage caused and make sure their futures get back on track to what they are supposed to be.