"Acid 2" is the sequel to the original "Metal Gear Acid", with the story set sometime after the events of the previous game. Like its predecessor, it follows an alternate continuity separate from the mainline "Metal Gear" series. The game uses a completely new cel-shaded graphic engine, replacing the darker graphics from the first game, and features an improved gameplay system. It comes with a PSP add-on called the Solid Eye, which is a folding cardboard box with specialized lenses that fit over the PSP's screen, creating a stereoscopic image.