The planet Antares consists almost entirely of a huge ocean, with the capital city towering over it. The ocean sustains the planet's life and is the source of its prosperity. The neighbor planet Betelgeuse, which has depleted all its resources, invades Antares and captures its capital city. Two years later, a special team of three sexy and skilled special agents is sent to reclaim the lost city. The team is led by the "crimson witch" Eo, who is also the protagonist of the game.
A-GA relies heavily on 3D cut-scenes to advance the story. The gameplay segments between the cut-scenes are quite short. During those gameplay portions, Eo is typically thrust into a closed area populated by enemies. Eo can jump, duck, and fight enemies in real-time with her lightsaber-like sword. There are also boss fights during which Eo will use a gun. The game's cut-scenes often contain explicit sexual situations: the girls, who already wear very revealing outfits, will sometimes undress, and also engage in sexual acts with various people.