Episode List: http://mlp.draconequus.com
After being trapped in an underground cavern, Twilight Sparkle finds another Princess Cadance, and learns that the Princess Cadance that Shining Armor is about to marry is actually an impostor. Twilight and the real Cadance escape in time to stop the wedding, where Cadance shows that the imposture is the Queen of the Changelings (Kathleen Barr). Queen Chrysalis reveals herself and admits to having fed off Shining Armor's magic. She is able to defeat Princess Celestia, and her horde of Changelings prevent Twilight and her friends from getting the Elements of Harmony. As the Queen revels in her victory, Twilight frees Cadance, who subsequently breaks the spell the Queen has on Shining Armor. Together, Shining Armor and Cadance use their magic to reform a magic shield that protects Canterlot and expel the Queen and her minions from it. Twilight forgives her brother and friends for not believing her suspicions about Cadance, and the rescheduled wedding goes off as planned.
Appears on these pages
22:04 MLP-FiM S2 E26 - A Canterlot Wedding, part 2