For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I: Being King Is Not All It's Cracked Up to Be
We meet Tom and Edward, two boys who somehow were born with identical appearances despite coming from different social circumstances and families. Tom has always dreamed of meeting the prince, and when he finally does, he could jump for joy. But when the boys decide to switch places, they're in for a much longer story than they expected.
Act II: The King Is Mad
Everyone believes that both Tom and Edward are insane. Nothing they can say or do makes anyone believe them, and they get deeper and deeper entrenched in their new roles. Tom will be trapped forever if he goes through with the coronation and becomes the new King of England.
Act III: Long Live the King
Finally, everything is fixed. Edward is revealed to be the true king. Tom gets to live the life he's always wanted, and everyone gets what they deserve. All in all, not a bad way to end the story.