Princess Celestia and Princess Luna are called on by Prince Dusk of the North Kingdom to aid him against a rising threat. To ensure both kingdoms remain trusted allies, Dusk proclaims that he will marry Canterlot's newest Princess, Twilight Sparkle, and in doing so will establish a permanent and trustworthy alliance between Canterlot and the Frozen North. However, secrets are being kept and there is a conspiracy against the two Princesses. Celestia and Luna find themselves on the brink of war as the Iron Empire steps forth to take control of Equestria, led by the hero-turned-conqueror, Metal Wing.

While Twilight Sparkle contemplates a future without her friends, Celestia and Luna suddenly find themselves being haunted by their past. The supreme conqueror, Metal Wing, plans to rule as Equestria's King, and seeks Celestia to be his queen. However, this hero-turned-conqueror also has a dark secret, one that goes back over one thousand years ago, a secret that is linked with Celestia and Luna's past.

At the same time, the Element wielders find themselves facing off against the Iron Empire's elite officers, the Iron Five, and must continuously put up with their shenanigans while the Princesses are trying to win the war.

This is a story of trust, love, betrayal, corruption and how absolute power can change even the kindest of Ponies into cold-hearted tyrants.