"No! It can't be! I just sent you back to the future!"

The first Back to the Future film ended with a sequel-ready finale, perhaps a show of writer/director Robert Zemeckis' optimism. As it turned out, that optimism was well-placed, as Back to the Future became the biggest hit of 1985. The subsequent home video release included a "To Be Continued" tag, and in 1989, it was.
Back to the Future Part II begins where the first film left off, with Doc Brown having returned from the year 2015 to tell Marty McFly and girlfriend Jennifer Parker that their future children are in jeopardy. The three load up in the time-traveling DeLorean (which now flies with garbage-fueled fusion power) and take off for Hill Valley, 2015. In the future, Marty and Jennifer's son Marty Jr. is about to be implicated in a crime that would bring disgrace to the McFly name. Taking his son's identity, Marty Sr. turns the tables on the bullying Griff (Biff's grandson) and his gang, then makes his escape on a skateboard-like hoverboard.

Problem fixed, Marty decides to make his time trip profitable by purchasing an old sports almanac, giving him sure bets for his 1985 self. Doc spots the cheat and tosses the almanac out, warning Marty not to mess with the space-time continuum. Unfortunately, an aging Biff has overheard the scheme, and while Marty and Doc are distracted, Biff steals the DeLorean for a quick trip back to 1955.

Marty, Doc and Jennifer return to 1985, but Hill Valley has been reshaped by the now-gazillionaire Biff, turning the once-pleasant town into a scummy gambling metropolis. In order to set things right, Marty and the Doc have to travel back to 1955 and get the almanac back from teen Biff, all the while being careful not to interfere with the events of the first film.

It was fast, frenetic, and for some, hard to follow, but Back to the Future Part II was another blockbuster hit for the budding franchise. Michael J. Fox, Christopher Lloyd, Lea Thompson and Thomas F. Wilson all returned from the first film, taking on even more roles in the sequel (Fox even played his future daughter, Marlene).

The film ended with a cliffhanger, promising yet another installment less than a year later. The two sequels were actually filmed back-to-back, a show of confidence from Zemeckis and Amblin Entertainment executives Steven Spielberg, Kathleen Kennedy and Frank Marshall. Their bet paid off, resulting in one of the post profitable trilogies in the history of movies.