Derpy Hooves is not your average mailmare. No, that's just a day job.
Her real work only happens when the mail's all gone.
When Equestria is threatened by forces from outside their universe, the royal sisters call upon Derpy, and her trusty sidekick, Colgate, to defend the world. Oftentimes she must take the battle back across the multiversal divide to stop the problems at its root.