Inspired by the Bioshock franchise.

Flurry Heart had an accidental outburst of magic sending Shining Armor and Cadence to what's left of Essence, but why don't they just teleport back to Equestria? I'll tell you, the walls absorb large amounts of energy in order to keep the power running forever. But someone is still alive, a man asks them to do something for him, if they do that they get to go on the last evac submarine and they can teleport back to Equestia.