Plot synopsis
In this sequel, Doc Brown uses his DeLorean time machine car to travel to 30 years forward to the future. While there, he finds Marty and his girlfriend Jennifer are now happily married, but their children are about to go to jail.
He returns to 1985 and collects Marty and Jennifer to take them back to 2015 to sort out the mess. Soon after they arrive, Biff, who is now an old man, recognises them. He steals their DeLorean car and travels back sixty years to 1955. While there, he hands his younger self a sports almanac containing all the sports statistics until the end of the century.
By winning millions on gambling, Young Biff distorts the spacetime continuum creating a new timeline. When Doc and Marty return to 1985, they find their hometown has become a living hell.They soon realise what has happened and decide to go back to 1955 to prevent Old Biff giving the almanac to Young Biff.
As they are about to return to the future, lightening strikes the car causing its time dial to malfunction. Doc Brown and his car arrive in 1885, leaving Marty stranded in 1955.
A lawyer then appears and gives a letter to Marty, which the Doc had written back in 1885. It explains where the time-machine car is hidden, so that Marty can use it to get back to 1985.
My comments
Soon after Doc and Marty arrived in the living hell on the alternate timeline in 1985, Doc draws a diagram of a branching timeline. He explains that when Biff came back, a new branch of the timeline must have been created.
Doc notes that if the original timeline had collapsed immediately, then Doc and Marty would have disappeared along with that timeline. He therefore concludes the new timeline could not have formed immediately on Old Biff’s arrival. It must have formed when a significant event had taken place, causing the original timeline to disappear.
Summary of time travel
As soon as Old Biff arrives in 1955, the timeline diverges slightly from the original one. However, it soon starts to converge back towards the original because Young Biff does not make any bets using the almanac. This means at this point, Doc and Marty are safe further along this timeline and Old Biff is able to return to 2015.
Young Biff though now has the potential to win millions. And when he does this, it is a significant enough event to cause the timeline to diverge away to a completely different future. Hence this film uses a double-well timeline, as shown in the first diagram below.
Another example of this is at the beginning of the film when Doc travels back from 2015 to 1985 to pick up Marty. They then travel forward to just before the events that Doc has witnessed. They eventually prevent Marty’s children from going to jail. This means they have caused the timeline to diverge as shown in the second diagram below.
This shows that the past and future can diverge, but only if a change is significant enough, hence the double-well timeline.
Category of time travel
Science fiction: a time-travel vehicle.
Model of time
Open past, open future with a double-well timeline.
Guyline for Old Biff returns to 2015 before the divergence occurs:
Guyline for when Doc travels back from 2015 to 1985 and then to just before the event he left:
