The Final Key is the second half of the story arc known as Triad, which began in Schism.

Schism ended with the Skolian Empire torn asunder by personal conflict within the royal family. With The Final Key, the Skolian Empire comes under all-out assault from its nemesis, the Euban Concord, who have undermined the Empire via subterfuge and assassination, leaving it ripe for conquest.

The Skolian Empire's only hope? A young woman barely out of her teens who hasn't even complete her training as a cadet.