"There are somethings in life that we won't forget….."

Soon after the receiving tickets for the maiden voyage of the Leviathan, the largest airship ever built, Twilight receives a disturbing letter, signed simply as "Mortis".

In order to find this killer, she enlists the help of Rook, a brilliant investigator. They must work together to stop the mad pony. However, Equestria's law system forbids such things. Vigilantes are subject to the death penalty.

Rook and Twilight must try and save the ponies on board the airship, all the while not being caught. An Mystery a mile in the skies of Equestria. Leviathan.