A narrative is a story or plotline. In Dahl's poem, he retells the Cinderella fairytale to make it scarier or more "gory," saying that the story that has come down to us is sanitized and "sappy" to keep children "happy."
In Dahl's version, Cinderella is locked in a basement with rats as her stepsisters go to the ball. When her magic fairy appears, this more assertive "Cindy" beats on the wall and demands to be sent to the ball. The fairy agrees, and Cinderella ends up dancing with the prince at the ball, clutching him so hard he "gasps." When the clock begins to toll, the Prince tries to hold on to the fleeing Cinderella, tearing off her dress so that she runs away in her underwear and loses a shoe.
In a twist on the original, one of the stepsisters takes Cinderella's shoe, flushes it down the toilet, and replaces it with her own shoe. The shoe fits the stepsister's foot, but the prince refuses to marry her, and instead chops off her head and kills the other stepsister as well. This upsets Cinderella, who appeals to her fairy to wed her to a decent guy. She ends up married to a kind jam maker and lives happily ever after.