After the events of Shifting Priorities, Skid is still in jail, and Twilight has gained wings and become a princess. However, Skid has gained a special phial that will enable him to get out of jail, and give him the perfect chance to take revenge. And Skid is going to make good use of that chance to both take revenge, kill Celestia, and regain his body. Patience has always been his strong point.
A Changeling from the attack on Canterlot needs to hide out in Ponyville.
Skid the changeling wants revenge on Twilight, and to kill Celestia. However, killing Twilight would be too simple.