Split Heart is one of many changelings who were under the control of a spell called the Hive Mind. But ,after her horn is injured, she finds herself free from the spell. Now she is left wondering how Chrysalis had managed to cast the Hive Mind. And who is the mysterious changeling that fogs her memories?
Also, in Canterlot, Shining Armor discovers side effects of the spell Chrysalis had cast on him. He hears voices in his head ordering him to hurt the ones he love. Although he fights them, the voices only grow louder.

Now, with no other choice, the captain of the royal guard must travel with Split Heart in order to find a cure and discover the truth behind the Hive Mind.