You have to put up with the muse's quirks, but sometimes she outdoes herself.
Giving a complete and detailed backstory, but being quite firm that it's all backstory, even hinting at the scene where it's told. A few hints of the beginning of the story proper. More hints about the ending. And with that I set out into the Valley Full of Clouds with my outline in hand.
It gets even more fun when the inspiration was another work's plot. On one hand, you have the rewrite, where you stick as faithfully to the (hopefully public domain) plot as you can, because it's part of the effect. If you retell Sleeping Beauty as a vampire story (unwise, it's a cliche), you need to adhere to the plot as faithfully as possible. (Probably even while you're rewriting a fairy tale no one will recognize -- sigh -- because it's not one of the top ten.)
Or if you just rip off the story for its notions. Then scrubbing off as much as you can is a theoretically simple affair.
It's when you look at a plot and think that it could have been done so much more effectively, or they missed the excellent resolution for the one they tacked on. The serial number usually can be scrubbed off without affecting the plot, but half the fun in that is seeing how much of the old plot you can lift. Except that it has to stand as a story on its own without being recognized. Which means sometimes you sacrifice the fun for the story.