Neifile wants to tell a story that is both delightful and morally good for her audience.
A worthy knight called Ruggieri de' Figiovanni decided not to waste his goodness in Tuscany so he road trips out to the court of King Alphonso of Spain.
But when he gets there, he's disappointed. King Alphonso is dishing out land and favors to the most unworthy people.
To make matters worse, he doesn't even acknowledge Ruggieri's good service.
So Ruggieri decides to leave. As a parting gift, the King gives him a mule.
Ruggieri's grateful for the animal; it's a long trip home.
Alphonso sends a "spy" along with Ruggieri. He's there to listen to what Ruggieri has to say about the king and then bring him back to the court on the second day to answer to the king if he's said anything bad about him.
Ruggiero's in a good mood until the mule refuses to pee when it should, and then pees in what should have been its drinking water.
He comments that the darn mule is just like the man who gave it to him as a gift.
Alphonso's spy took note of that one.
On the second day, the spy delivers the message that Ruggiero has to turn back.
When they arrive back at the court, Alphonso asks Ruggiero what he meant by comparing him to the mule.
Ruggiero explains that like the mule, he does things backwards. He gives gifts where he shouldn't and withholds them when he should give.
King Alphonso explains that it's just Fortune and not Alphonso's stinginess that deprives the knight of reward. And he can prove it.
He sets two chests before Ruggiero and asks him to choose one, Monty Hall style. One has all the king's jewels and the other has dirt (Ruggiero can't see what each one contains).
Ruggiero chooses a chest and it turns out to be the one filled with dirt. King Alphonso has a good laugh.
You see, he tells Ruggiero, you just have bad luck. Don't blame me.
But he gives Ruggiero the chest full of treasures anyway to defy the power of Fortune—and because of Ruggiero's good service.
Ruggiero leaves this time pretty pleased at Alphonso's crazy generosity.