The Hero's Journey (Monomyth) can be mapped onto Three or Four Act structure. But excatly how this happens is variable.
a) Often a story will not proceed past Ultimate Boon. The Atonement with the Father, Apotheosis and Ultimate Boon form Act III. This is often the case when there are three, not four, Core Challenges.
b) With Four Core Challenges, the story often proceeds into the Return, thus from Refusal to Freedom to Live and Afterlife we have Four Acts.
c) Each challenge is delicately resolved in turn. In fact, each minor cave within each major cave represents one of the Challenges.
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