Perhaps you're thinking of a story with a dragon that has a weakness like:
* Fire: Some dragons are vulnerable to fire, especially if it's a different kind of fire than their own breath.
* Cold: Certain dragons might be weak to cold, especially if their element is fire.
* Magic: A specific type of magic or magical artifact could be the key to defeating a dragon.
* Specific objects: A dragon might be vulnerable to a particular object like a rare herb or an enchanted weapon.
If you could provide more context about the story you're referring to, I can help you pinpoint the dragon's weakness!