Possible Clues:

• Although you may not beleive it, it's not the story of Tom Thumb
• That yarn is the height of improbability
• All told, it's the height or improbability
• All, told, that's the height of improbability
• High storey heard as not necessarily 15
• High romance?
• Cock-and-bull story
• Fishy story
• That's incredible!
• Imaginative story