When Twilight decides to study the magic of non-pony races she inspires a Changeling sorceress, named Amber Fire, to break out of a Las Pegasus prison and travel to Ponyville. Meanwhile, Lyra Heartstrings is returning from Las Pegasus distraught over being single for the first time in years. The two wind up sharing a train car to Ponyville, and it becomes clear to Lyra that this "Unicorn" knows something about humans. Determined to learn everything her new friend knows Lyra winds up getting closer and closer until eventually she learns not just one but two deeply hidden secrets.

(There isn't actually a Pony Souls I, the title is just a joke at bad titles and a hint at the cross over elements. Retitled because I thought of a good one at last.)