Commentary
While not nearly as great as the original, Back to the Future Part II is nevertheless a very entertaining film. In some ways it is better in fact. For instance, the sequel involves travel into the future. Also, the sequel involves much more in the way of time travel paradoxes than the original. On the other hand, much of the plot of the sequel is essentially a copy of the original. For instance, Back to the Future Part II has a skateboard chase scene that's essentially the same as the scene from the original movie, but set in 2015 instead of 1955. Also, the last part of the sequel shows many of the same things happening as in the original, when Marty and the Doc return to 1955.
Plot Summary There are three parts to Back to the Future Part II, each part set in different years.
2015: In the first part of the movie, the Doc takes Marty, Jennifer, and Einstein to the year 2015 where he tells Marty and Jenny that their children are in trouble. They must stop their son Marty Jr. from collaborating with Griff (Biff's grandson) and his gang in order to avoid a 10-year jail sentence. Marty disguises himself as his son, who looks just like him, and ends up involving the gang in a hoverboard chase through the Hill Valley town square, much like in the original movie. The gang crashes into the city hall and gets arrested instead of Marty Jr. Meanwhile, Jenny inadvertently ends up meeting her future family at her future home and she and her future double both faint when they encounter each other.
1985: Done with their business in the future, Marty and the Doc return home with Jenny and Einey, but to their dismay, they find everything has changed for the worse. Now Biff owns Hill Valley and is married to Marty's mom, Marty's dad is dead, and Doc Brown is committed. The Doc and Marty learn that the cause of their problems is that Biff's future self in 2015 had handed a copy of Gray's Sports Almanac, with results from every major sporting event from 1950 to 2000, to his younger self in 1955. Thus, they need to go back to 1955 to get the almanac from him and restore the present.
1955: Doc and Marty return to Nov. 12, 1955, the same day as the Hill Valley thunderstorm from the original movie, which turns out to be the day that Biff from the future handed his younger self the almanac. After lots of aggravatiion, Marty finally recovers the almanac and burns it, restoring the original timeline. But then Doc gets struck by lightning while hovering in the DeLorean and ends up in the Old West in 1885. Marty meets up with the younger Doc from 1955 right after he sends his double back to 1985.
Time Travel Paradoxes As I mentioned, Back to the Future Part II is full of time travel paradoxes, much more so than the original. Here are some
Alternate Timelines: The second part of the movie is a horrible alternate timeline reminiscent of It's a Wonderful Life in many ways. Because Biff obtains information about the results of future sporting events, he is able to amass millions of dollars, becoming rich and powerful, ultimately buying out Hill Valley, killing Marty's father and marrying his mother. As a result, Hill Valley in 1985 turns into a hellish place, rampant with crime and gambling, which is now legal in California. Marty and the Doc restore the timeline in 1955 when they recover and burn the almanac. The other timeline they fix is the one in which Marty's children get arrested in the future.
Changing Photos: As timelines are restored, photos and such from the future change, much like in the original movie.
Doubles: The young Jenny from 1985 encounters her older self in 2015, along with an older version of Marty. Also, In 1955, both Marty and the Doc encounter their own doubles from the first movie.
Futurism
Many technological advances are predicted for 2015. These are very bold predictions and I'd be impressed if even one of them comes to pass by 2015!
Fusion power
Hovering capabilities for cars, skateboards, and other devices
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Title: Back to the Future Part II 
