The Ponyville dam has stood for decades, holding back the immense pressure of the waters. But everything must fade with time. Restoration work is under way on the solid monolithe. But something goes wrong; the dam cracks, and threatens the workers below with the immense fury of the water held back behind it.Twilight attempts to save the workers with her magic; but not even the most powerful unicorn can do it alone. After a vigorous battle with the dam wall, she passes out from over-exertion.

But; when she wakes up in hospital the next day, she is told that a mysterious Unicorn with a black mane held the dam together long enough for the rest of the Unicorns to arrive and take care of the situation. And then she promptly disappeared into the Everfree forest without a word.

Who was this mysterious unicorn? Why did she retreat so suddenly?

The answer will surprise everyone.