In a world not completely unlike our own, there is a country called the Canopy Kingdom. Vaguely similar to 1940s America, the Canopy Kingdom is home to millions of humans, fish-like Dagonians, animal-like Ferals, and the giant-sized Gigans. This kingdom is blessed with a very socially-conscious royal family, a strong military, and amazing technology.
However, it is also cursed. Every seven years, a strange jewel called the Skull Heart appears. The Skull Heart is said to grant the wish of any woman who can grasp it...but if their wish or their heart is impure, they will be transformed into the Skullgirl, a powerful, necromantic menace that could one day become powerful enough to destroy the world. So far, there have been hundreds of Skullgirls, each one brought down in a horrific war more bloody than the last.
Seven years ago, Queen Nancy of the Canopy Kingdom became the Skullgirl. Her reign of terror was ended by the Canopy Kingdom's powerful military and the work of the Anti-Skullgirl Labs. After the Labs were disbanded, they went rogue, fearing a new Skullgirl's appearance and delving further and further into forbidden weapons research.
Now, a new Skullgirl has appeared. She appears weaker than Queen Nancy, but the Canopy Kingdom will take no chances with a Skullgirl.