Act 1 and Act 3 act as mirrors of each other. Act 1 sets up the characters’ goals and hints at the transformation that is coming in Act II and culminates in Act 3. At the end of the book, the protagonists must return home, literally or figuratively, changed by their experiences and healed of the wound that plagued them in Act 1.

Say your protagonist’s wound is that they deeply desire belonging. They spend all of Act 2 looking for it in all the wrong places. Finally, through their many failures, they discover that home is where the heart it and find peace with whom they are inside.

Or take the example of Bilbo Baggins. He is happy with his life, dislikes company, especially of the rowdy kind. He literally leaves home, follows some boisterous dwarves on an adventure that challenges and changes them. By following the dwarves on the adventure, Bilbo Baggins broadens his understanding of the world and returns home with a new perspective. He returns to the literal place he started treasuring and even craving adventure.

In The Searchers, Ethan Edwards has returned from fighting in Mexico. He seeks out the comfort of family. However, fate intervenes and everyone except his niece is killed. In Act 2, he sets out to reunite his broken family. After several years and many misadventures, he finds her. She’s not what he expects, and he considers destroying the family he’s struggled to preserve. But his adventures have hardened his resolve, and the friends he met along the way taught challenged his deep-rooted biases. Instead, Edwards chooses family and completes his mission by bringing Debbie home.