Ikaruga is a shoot 'em up developed by Treasure. It is the spiritual sequel to Radiant Silvergun (1998) and was originally released in Japanese arcades in December 2001. The story follows a rebel pilot named Shinra as he battles an enemy nation using a specially designed fighter called the Ikaruga which can flip between two polarities, black and white. This polarity mechanism is the game's key feature and the foundation for its stage and enemy design. All enemies and bullets in the game are either black or white. Bullets which are the same color as the player are absorbed while the others will kill the player. The game features both single-player and cooperative modes.