The hero, armed with the mentor's gifts, agrees to face the journey. This is the turning point between Act One and Act Two, the crossing from the ordinary world into the special world. The hero is wholeheartedly committed and there is no turning back.
According to Christopher Vogler's The Writer's Journey: Mythic Structure, crossing the first threshold is often the result of some external force that changes the course or intensity of the story: someone is kidnapped or murdered, a storm hits, the hero is out of options or pushed over the brink.
Internal events might also signal the crossing of a threshold: the hero's very soul is at stake and he makes a decision to risk everything to change his life, Vogler writes.
Heroes are very likely to encounter threshold guardians at this point. The hero's task is to figure out some way around these guardians. Some guardians are illusions and the energy of others must be incorporated by the hero, who realizes that the obstacle actually contains the means of climbing over the threshold. Some guardians simply need to be acknowledged, according to Vogler.
Many writers illustrate this crossing with physical elements such as doors, gates, bridges, canyons, oceans, or rivers. You may notice a clear shift in energy at this point.
A tornado sends Dorothy to the special world. Glinda, a mentor, begins teaching Dorothy the rules of this new place, gives her the magical ruby slippers, and a quest, sending her over a threshold where she will make friends, confront enemies, and be tested.
Tests, Allies, Enemies
The two worlds have a different feel, a different rhythm, different priorities and values, different rules. The most important function of this stage in the story is the testing of the hero to prepare her for the ordeals that lie ahead, according to Vogler.
One test is how quickly she adjusts to the new rules.
The special world is usually dominated by a villain or shadow who has set traps for intruders. The hero forms a team or a relationship with a sidekick. She also discovers enemies and rivals.
This is a "getting to know you" phase. The reader learns about the characters involved, the hero accumulates power, learns the ropes, and prepares for the next phase.