It takes place after Twilight forgives Trixie for what she did in the episode "Magic Duel", and Spike doesn't buy it. Twilight learns from her friends how Spike really feels for her and now she has to fix it.
Spike ends up being transported mysteriously into a strange, but familiar land, there he meets Steven Universe, a magical boy (half magic, from his mom's side) cool and nice.
There's been an accident, and something awful has happened to our favorite dragon. After recovering, our dragon leaves his friends and family to solve and stop the organization that has caused him this pain. Via hunting, revenge, and bloodshed. Yay!