I guess the first question is really this: What is the Flux Capacitor?
Easy:
…it’s what makes time travel possible.
And that’s all you need to know.
What a great bit of storytelling! The entire Back to the Future franchise rests on the ability of Marty McFly and Doc Brown to travel through time in their suped-up DeLorean. But do we really need to know how time travel works? Nope. We just need to know that it does.
Therefore the Flux Capacitor makes time travel possible and it also makes the story possible. All without being explained in the slightest.
So the real question of this post is: What makes a story possible? What is it that a plot relies on to make all the action which follows it make sense? If you are writing a story, do you know what is special about your story which allows it to happen?