Rainbow Dash is angry and suspicious. Deciding to see if her suspicions are truth or just paranoia, she decides to confront the 'problem' at it's source.

Part of the Cornerverse-MLP Universe.
Rated Teen for swearing, talks of LGBT stuff, and mentions of sex. Not sure if the 'sex tag' is appropriate for this but it's more direct than previous innuendos and acknowledgements so better safe than banned.
You don't really have to read the prequel, but it is technically a part of it.