When Marty McFly returns home to 1985 after his brief trip to 2015, he finds everything has changed into a bleak, dystopian horror where the town of Hill Valley is ruled over by billionaire Biff Tannen. The Biff of 2015 stole Doc Brown's time machine and brought a sports almanac back to his teenage self in 1955, giving him the winner of every sporting event for 40 years.
To restore everything and get back to the future, Marty must obtain the almanac before Biff can use it. But the book is in 1955 and that means Marty must dodge not only Biff but also himself - the Marty McFly struggling to escape 1955 in the original "Back to the Future" movie.
It's a tremendous and audacious idea. So many sequels to movies are really remakes, just broadly changing character names or settings, but "Back to the Future Part II" is the only sequel that takes place during its own first film. We constantly see the "Part II" Marty appearing in scenes from the first film alongside Marty from the first film, often coming very close to disrupting events or providing an alternative take on what was happening.
The idea and the execution is so good that you could applaud - yet the film fared less well than either the first or the third films. It was made four years after the first and the idea of Michael J Fox as a teenager had already been strained, but arguably what weakens this movie is that being in the middle of the trilogy it lacks the huge finishes of its companion films.
It's best to regard this instalment and the next as one movie. They were certainly written together and made back to back - so perhaps you should try watching them one after another too.