Twilight visits the Crystal Empire for her first royal summit following her coronation as a princess of Equestria. Sunset Shimmer, a rogue student of Princess Celestia, emerges from a mirror portal and steals Twilight's crown, which contains the Element of Magic. After a chase through the castle, Sunset goes through the portal with Twilight's crown. The other princesses explain that the portal leads to an alternate world; as the other Elements of Harmony are unusable without the crown, Twilight is tasked with retrieving it from the other world before the portal closes for thirty moons. Despite Celestia's insistence that Twilight must travel alone, Spike follows her into the portal. (Source: Wikipedia)