They are all against having any other type of life form living on their planet. That's where you come in. Water is essential for life. Your goal is to create lakes on the planet and to keep the water flowing while fending off those four foes. You do that by dropping different-shaped blocks out of the sky, a la Tetris, and planting walls to keep the water in.
Nothing is particularly impressive.
The graphics are vanilla, and the sound is annoying - the game is constantly screaming out your lake status. The touchy game play will frustrate you, and at times, you'll find yourself holding back from tossing the game out the window. But Aqua Aqua, like its much older cousin Tetris, has a knack for getting you hooked and coming back for more. If you liked Tetris, give it a shot. If not, take a pass.