At about the 20 percent point of the story, corresponding to the end of Act I in the three-act drama structure, there’s the First Reversal. Something happens to transform the Hero’s new situation into a new desire and a change of plans. This will set the Hero’s external motivation for the rest of the novel, and the action is underway.
We see the Hero in the World of the Common Day from which he is summoned via a Call to Adventure from a Herald to the threshold of adventure and eventual departure into the Magical World. He refuses the Call at first, then finally accepts, prepares for the Journey, and encounters a Threshold Guardian, who warns him to turn back. The Hero then enters the Mythological Woods.