Metal Gear Acid is not canon with the main Metal Gear series and although the plot contains Solid Snake, it is just a side-story.
Terrorists have hijacked a flight containing an important politician, Senator Hach, and are demanding "Pythagoras" in exchange. Pythagoras is a research project being conducted in South Africa so the US Government begins an investigation of Pythagoras in an attempt to identify the terrorists. After the Moloni government (country where Pythagoras is being conducted) refuse to help the US, they send in a covert special forces team but they are beaten by armed resistance. They resort to sending the previously retired Solid Snake. Solid Snake is sent to the facility in Africa to infiltrate it and find out about Pythagoras so that Senator Hach can be saved.
During the game you will encounter Teliko, who was apart of the special unit that was sent in before Snake to try and deal with the situation however things got tough and she was the only one to make it out alive. She helps you in your mission to find the Pythagoras - which is revealed to be the new Metal Gear - together, but during mission you are cut off by a brute like man known as Leone who insists on killing you. However, he soon changes his mind and decides to join Solid Snake as well.
Because the game was just a side-story, it's gameplay was also nothing like that of the previous games. Fights were described as 'turn-based strategy' which meant that in a fight, you would have cards which dictated attacks and movements. You would collect cards in a deck throughout the game, but in a fight you could have one 'hand'. This consisted of 6 cards which you would choose previously.
This gameplay worked well for providing link battles between two PSPs where both players would have to collect a certain number of diskettes before the rival player.