Twilight Sparkle-- a nerd, book pony, and now a Princess of Equestria-- is tasked by Princess Celestia to hold in her pee for an entire week. Fortunately for her, holding in her pee and keeping her bladder full is something she's familiar with, but the stretch of seven whole days is something she'd barely even gotten halfway to in her previous and personal ventures. Unlike other mares, however, her external urinary sphincter isn't the only form of control she has over her bladder... and with a helpful gift from Celestia herself, she may just be able to reach an entire week.

...And then realize that was only the beginning of her adventure.

Original cover art belongs to Bugplayer
This is a fetish-centered story that's focused on full bladders and pee desperation.