“ A middle is that which follows something as some other things follow it.”
(Poetics, Part VII)
Act One was all about the Stepping Stone events necessary to introduce the hero and his world, and then to launch him into the PLOT of the movie. So it ends with a big reversal, a Point of No Return that pulls him from the world of the equilibrium, and deposits him in a new one, on the path toward the new objective that provides the dramatic question the rest of the story will resolve.
In Act Two, we will experience all the hero does to achieve that objective, the obstacles that stand in his way, and the opportunities he has to change and grow while attempting to overcome them, until that pursuit ...