The title says it all, but to elaborate, Twilight is experimenting with a shapeshifting spell and it goes badly, resulting in herself and any pony else with her being stuck as humans (Or another rarely seen species if human doesn't appeal to you.) Twilight keeps her magic, but must learn how to cast it in her new form before she can even begin to try returning them to normal.
The title says it all, but to elaborate, Twilight is experimenting with a shapeshifting spell and it goes badly, resulting in herself and any pony else with her being stuck as humans (Or another rarely seen species if human doesn't appeal to you.) Twilight keeps her magic, but must learn how to cast it in her new form before she can even begin to try returning them to normal.