Twilight is at the end of her rope. Spike has since turned 18 but acts like he's still a child. He lies about where he's been, he lies about what he's doing, he lies about everything and doesn't seem to understand the harm of it.
After trying everything she can think of, Discord comes in and offers her a rather extreme solution. Though hesitant, Twilight feels she's out of options. She accepts.
Now Spike has to deal with a female body. A plain one that keeps growing more and more girly with each and every white lie he tells. Will he learn not to lie? What happens when he gets too used to her new body that she begins to think of herself as a she? Also what's up with the way Rumble is looking at her?
The sex tag is only because given the nature of the story, it'll mention female body parts. I do not plan on making this mature, and the characters are all 18 only as a 'Just in case' measure. Again because it's impossible to do this story without mentioning female body parts. The worst I want is maybe the occasional boob poke, but nothing more.