a good story
You say this when you know someone is lying and you just want to tell them that you know they're full of crap.
When my friend said he got a perfect score on his math test, I said, 'A good story? Bro, you got a 52.'
I said 'A good story?' when my brother told me he won a million dollars in the lottery.
My mom told me I was the best student in the class and I said, 'A good story? I got a D in math.'