With an ancient, magical horseshoe permanently stuck to her hoof by accident, Sweetie Belle's life is almost immediately turned upside down. She discovers her sister isn't the mare she thought she was. Her parents aren't who she thought they were, and that the key to unlocking everything is stuck to her hoof.

Now she and the Crusaders must venture deep into a forgotten underworld to try and remove the horseshoe from Sweetie's hoof, with the Mane Six protecting them. Unfortunately, they do not have long to accomplish this, as the key within the horseshoe is powered by the wearer's soul and as a filly, this leaves her only forty five days before her soul becomes trapped in the crystal forever.