Twilight went alone to the Crystal Empire at the request of Princess Celestia, but unfortunately, she failed to save the city from King Sombra, and now she is trapped by King Sombra's trap and needs to find a way out.
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.