"Metal Gear Ac!d" was published & developed by Konami Computer Entertainment Japan for the PlayStation Portable.
Instead of the traditional 3rd person stealth/action gameplay of the series, Metal Gear Ac!d is turn-based, with all movements/actions decided using a collectible card game mechanic. The player has a pool of cards (their deck) which is randomly drawn from & used to dictate the currently available actions - any card can be discarded to be used for movement. If the player wants to shoot at an enemy, they must have a weapon card available and ammo cards too. Each card has an action point cost associated with it to play, when the active character (including the player) runs out of action points, the 'next' character begins moving/playing cards etc.
As the player progresses through the game, they earn/buy/find new cards, unlocking new special attacks, weapons, ammo, health etc. Notably, some cards in the game are associated with previous characters/events in the series.
The game features local ad-hoc multiplayer support for 2 players.