The game is a spin-off title of the Hyperdimension Neptunia game series featuring Noire as the main character. Unlike previous Hyperdimension Neptunia games, Black Heart is not set within the world of Gamindustri, but instead within a similar but separate world known as "game market". The goddesses of each of the four nations battle against one another for hegemony over the world day by day, however just as Noire is one step away from complete domination, the goddesses' powers are robbed by an unknown force. In order to save the world from destruction, the four goddesses cooperate with one another with the aim of unifying Game Shijoukai.
Source: Wikipedia, "Hyperdevotion Noire: Goddess Black Heart", available under the CC-BY-SA License.