Launching alongside the PSP, Metal Gear Acid made several changes to the well-established Metal Gear franchise. Rather than being an Action game, Acid is a Turn Based Strategy and Collectible Card game, a huge departure from the previous titles. But while the method of playing the game had changed, the fundamental gameplay had not. Players were still expected to use whatever means necessary to avoid enemy detection and complete their objectives all the while encountering over the top bosses, unravel a complex plot and stop the latest iteration of Metal Gear.
Metal Gear Acid allows players to construct decks out of the cards they have collected, either from a completing a mission or buying a booster pack, that enable Snake to do various moves prior to any mission. With over 200 cards to collect, players have many options when it comes to constructing a deck. The cards themselves are organized into different sets, that have different themes spanning the entirety of the Metal Gear universe as well as some other games Kojima has produced over the years. Each card has a different effect: some conjure the images of past bosses and allies, such as Gray Fox or Metal Gear REX to help defeat your foes, while others heal Snake, like the trusty Ration card. Many of the cards represent the many weapons seen in the Metal Gear franchise, including the SOCOM from Metal Gear Solid or the Mosin-Nagant from Metal Gear Solid 3: Snake Eater.
Metal Gear Ac!d was followed up by Metal Gear Ac!d 2 (also on the PSP), but it's not a direct continuation of the original.