Friendship has always made Starlight Glimmer uncomfortable—since Sunburst—but the friendship lessons keep coming. When Celestia and Moon Dancer's great aunt aim Starlight at the enigmatic Prince of Equestria, to teach him a lesson, unexpected feelings of friendship and love rear their ugly head. It really bugs her. It feels like she's being forced to take the advanced course in friendship when she's just learning what it even means; could it be more than friendship!? Whether she likes it or not, her choices about him will save or doom Canterlot—and change her life forever.
Prequels? The story is written such that you should not need to read the prequels. Your fun will be doubled, however, if you do.
About warning tags: Nothing explicit, but a warning nonetheless.
Additional MLP series characters: Sunset Shimmer, Twilight Sparkle, Spike, Shining Armor, Fizzlepop Berrytwist (before becoming Tempest Shadow), Sunburst Mortorboard, Grampa Gruff, a diamond dog,, and Moon Dancer. I could add a dozen more, but introducing them before they show up during syndication is spoilery.
Artwork by Syrupyyy at https://twitter.com/SyrupyyyArt