Today is the day, the distant future, to which Marty McFly travels in Back to the Future Part II, hurtling through time with Doc Brown to October 21, 2015. As predicted in the first film, Marty sees some serious shit — hoverboards, Pepsi Perfect, Jaws 19 playing in the local Holomax Cinema. Paradoxically, if Marty were to actually arrive today he’d find Back to the Future Part II re-released in cinemas instead, depicting the story of the day he traveled to October 21, 2015. He’d sit in the theater and have his recent past recounted and his impending timeline spoiled, which is an obvious time-travel no-no. His actions in the future would be influenced by the movie depicting his actions in the future, which would in turn change the 2015-set scenes of BttF2, which would in turn jeopardize Marty’s presence in that very theater, which would in turn jeopardize our ability to hypothesize about Marty’s presence in that very theater, which would in turn [head explodes].
The actual plot of Back to the Future Part II isn’t actually much simpler. If there are Ten Basic Ideas about time travel — meeting yourself, erasing stuff from existence, etc. — then three of them made it into the first movie and all ten of them were crammed into Part II, leaving Part III to differentiate itself by pretty much not being a time travel movie. But simple time paradoxes (paradoxi?) are for wimps — let’s have Michael J. Fox play a billion different roles, including three versions of Marty McFly! So silly!