The theme of the jam is A Delusion
The protagonist's main objective must be based on a belief that is clearly and obviously false (within your game's world).
The player must know from the beginning (or near the beginning) that the belief is false. ALSO, all the other characters in the story can clearly see that the belief is false. Yet the main character stubbornly keeps the belief despite all evidence and reason to the contrary.
The main character could realize the belief is false at the very end (but not before), or could not and continue believing it through the end of the game.
A classic example of this is the story of Don Quixote, although there are many other different ways the story could go.