A new kind of evil and destruction threaten not the Elements of Harmony, but the four ponies with control of the classical elements- fire, earth, water, and air.

When Celestia and Luna pass away, Princess Celestia puts the Mane Six in charge of running the kingdom. She guides them through her dead words and leaves important magic for them to use while ruling Equestria.

When the Mane Six are out of town saving Equestria, the Cakes ask Discord as a last resort to babysit the twins. When you have Discord around, you of course never know what will happen.