Back to the Future Part II starts on October 26, 1985. Dr. Emmett Brown comes riding his flying time machine. He goes to Marty McFly and tries to convince him and his girlfriend, Jennifer Parker, to go back to the future to help their future children. When they arrive on October 21, 2015, Dr. Brown knock Jennifer unconscious. He informs McFly that she should not know too much about the future. He then instructs McFly to pretend as his son and tells him to decline an offer to join a robbing spree with Biff Tannen’s grandson, Griff.
When Marty rejects Griff’s proposition, the two men engage in a fight. In the end, Griff and his group get arrested. Before he goes back to Dr. Brown, Marty buys a sports almanac of the years 1950 to 2000. Doc finds out about it and he scolds Marty regarding the consequences of messing up with the future. The police arrive and tell them that they have taken Jennifer to her home in the year 2015. Little they do know that Biff has overheard their conversation.
Jennifer regains consciousness in her future home. She learns that in the future, Marty gets involved in a vehicular accident and he also loses his job when he agrees to do a shady deal with his co-worker. When she tries to leave the house, Jennifer from 1985 comes face to face with her future self. Both Jennifers from the past and future faint. Marty and Dr. Brown find Jennifer and Biff sneaks into the time machine to bring the almanac to his younger self in the year 1955. Marty, Dr. Brown and Jennifer go back to 1985 not knowing what Biff did.
The trio are surprised to find how 1985 has changed dramatically. Biff is a wealthy and corrupt man, running Hill Valley with chaos. In addition, Marty’s father has been killed in 1973 while his mother is coerced by Biff to marry him. They also find out that the Dr. Brown from the now 1985 is locked up in an asylum. Marty and Doc discover that Biff has brought the almanac from 2015 to the past on November 20, 1955. Because of this, Biff tries to kills Marty but he manages to escape with Doc and they go back to 1955.
Marty observes how Biff from 1955 has gotten the almanac. He trails Biff until they come to the high school’s dance. Marty and the 1955 Biff do a cycle of stealing the almanac from each other until Biff finally manages to hit a manure truck. Marty lights the almanac on fire. Everything then goes back to the way it was in 1985. As they are about to return to 1985, a lightning strikes the time machine and it disappears. Suddenly, a courier arrives with a letter for Marty. When he opens it, he learns that it is from Doc who is transported back to 1885. Marty runs back to town to find Dr. Brown from 1955 who has just helped the original Marty return to 1985 moments before McFly arrives. The movie ends with Dr. Emmett Brown passing out when he sees Marty again.