'''''Metal Gear Acid''''' (stylized as '''METAL GEAR AC!D''' on the logo) is a Franchise/MetalGear spinoff released for the {{PlayStation Portable}} in 2004. The first game in the franchise released for the PSP, rather being a stealth-action game like its console counterparts, Acid (or MGA) is a turn-based strategy game featuring a card-based combat system in which the player's weapons, equipment and abilities are determined by the player's deck. While the story is set in an alternate continuity and has no bearing in the main ''Metal Gear Solid'' canon, the player's cards feature characters, items and other elements from the first three MGS games, as well as other KCE Japan titles released at the time.

In 2016, a terrorist group has hijacked a plane using a chemical weapon, threatening the lives of its passengers, with U.S. Presidential candidate Viggo Hach among the hostages. The terrorist demand that the U.S. Government hand over the "Pythagoras", a top-secret project developed in the South African country of Moloni. With the Moloni government refusing to cooperate, the US sends FOXHOUND member Solid Snake to infiltrate the Lobito Physics and Chemistry Laboratory in Lobita, where the Pythagoras is being kept. With the help of Teliko Friedman, a member of the FBI's Hostage Rescue Team that was sent to retrieve the Pythagoras, Snake learns about the true nature of the Pythagoras.

A sequel was released in 2005 titled '''''Metal Gear Acid 2''''' (or '''METAL GEAR AC!D[[superscript:2]]'''), also on PSP, featuring more than twice the amount of cards, cel-shaded graphics, a new Arena Mode, and an add-on for the PSP called the Solid Eye that allows the player to view the game in 3D.

Set sometime after the events of the first ''Acid'', Snake (now a member of a Resistance force) reluctantly agrees to infiltrate a facility located in an island off the coast of North America, owned by a military contractor named [=SaintLogic=] Inc. There, he finds out that [=SaintLogic=] has been involved in the development of a new Metal Gear tank.

Following the release of both Acid games, along with ''VideoGame/MetalGearSolid: Digital Graphic Novel'', Konami decided to produce a more traditional Metal Gear for the PSP titled ''VideoGame/MetalGearSolidPortableOps''.

!!This game provides examples of the following tropes:
* AbsoluteCleavage: Take one look at Takiyama...
* ArbitrarySkepticism: Snake isn't convinced Alice is actually psychic, even though such powers exist in the Franchise/MetalGear universe. [[spoiler:He turns out to be right... at least partially. Alice was really just familiar with the layout of the base, but there was some sort of reincarnation of children involved. [[VoodooShark It's complicated.]]]]
** Snake, to Venus' dousing, for the same reasons. He's also a bit skeptical of the second-to-last Boss's psychic powers. Until he walks on the ceiling.
* ArtShift: The in-game graphics in the first ''MGA'' were not that much of a departure from the mainline ''MGS'' games at the time, despite the use of anime-like character portraits. ''[=MGA2=]'' on the other hand, uses a cel-shaded style that makes it stand out from the rest of the series.
* AwesomeButImpractical: A rare for-the-villains version. Chiagdiel, a Viet-Cong psychic ninja with a grenade launcher, whenever he walks on the ceiling, is a stationary target and easy to whack with, say, a rifle. Or a good old fashioned handgun. Hitting him with even a pistol will cause him to fall off and take extra damage.
* BigBadEnsemble: Leone, [[spoiler:William Flemming/Gary Murray]] [[spoiler:Lena Arrow]] and [[spoiler:Alice Hazel]].
* BrainwashedAndCrazy: [[spoiler:Venus. For starters, she attempts to kill Snake nearing the end because Wiseman ordered her to. It is also heavily implied that she couldn't refuse his orders as he removed her free will.]]
** [[spoiler:Snake and the other test subjects were this, but gained amnesia. Venus was one of the more "successful" ones.]]
* ClearMyName: [[spoiler:Leone's]] last words, [[spoiler:"It wasn't me,"]] are to clear [[spoiler:his]] name.
** Snake also has to do this in [=MGA2=]. Well, technically - Dalton's blackmailing him into investigating [=SaintLogic=], and if he does so, he'll get pardoned and will be a legal immigrant to the US.
* ContinuityNod: Nearly all of the card decks, for one. You can even shoot guns that were in the original Franchise/MetalGear games - in sprite form, no less.
** On a side note, some weapons (like the PTRS/PTRD, RPK, et cetera) made their way into ''VideoGame/MetalGearSolidPeaceWalker''. The [=XM8=] and [=XM25=] also made their way to ''VideoGame/MetalGearSolid4''.
** The Tobidacid is referenced in ''[=MGS4=]'' during a codec conversation with Otacon, where it is said to be one of Solid Snake's childhood toys.
** Venus and Teliko are recruitable, hidden soldiers in ''Portable Ops''.
** Dr. Koppelthorn, the main antagonist in ''[=MGA2=]'', was previously mentioned character in ''Ghost Babel'' (where his name was often mentioned by Mei-Ling), as well as in ''[=MGS2=]: Substance'' (where he is the inventor of a VR simulation machine in the Snake Tale "External Gazer").
** In ''VideoGame/MetalGearSolid3'', rations were used to replenish stamina, rather than to heal as in the other games. However, in the ''Ac!d'' games, since there is no stamina, rations heal again. The description for the "Ration" card in the second game says that in the old days, soldiers used to think that rations restored stamina.
* DefogOfWar: There are multiple cards dedicated to this, though they all have certain drawbacks, like having a high cost draw.
* TheDogBitesBack: [[spoiler:After Vincent is defeated by Snake and Venus, he learns that Rodzinski is leaving the facility on his getaway chopper and leaving the security forces to take the blame. After a particularly big KickTheDog moment from Rodzinski, Vincent orders his soldiers to shoot him down, with the soldiers obeying the command without hesitation, presumably agreeing with him about why they should do so.]]
-->'''Security Forces:''' Major! We've located Mr. Rodzinski! He's about to leave in a chopper, sir.\\
'''Vince:''' Put him on.\\
'''Rodzinski:''' What do you want? Wait. Vince, is that you?\\
'''Vince:''' Mr. Rodzinski. What are you doing?\\
'''Rodzinski:''' I'm getting the hell off this island!\\
'''Vince:''' But what about Koppelthorn?\\
'''Rodzinski:''' You think I give a damn about that lunatic?! The ICC has decided to carry out their investigation! My deal with those goddamn politicians was worthless! After all the money and assets I invested in this, they abandoned me at the first sign of uncertainty! The ICC is sending an investigation team to the island next week. At this point, they won't let me go even if I get them the Lucinda File. The deal is off, and now I'm facing criminal charges. If I'm tried under the Serena national judicial system, I could be executed!\\
'''Vince:''' ... What are you going to do?\\
'''Rodzinski:''' Run like hell! What other options do I have? I can't set foot in this country ever again. ...I never believed this day would come.\\
'''Vince:''' So you're leaving this whole mess here with us?\\
'''Rodzinski:''' Spare me the sad story, Vince. You're the big boys. Take care of yourselves for once!\\
'''Vince:''' ...\\
'''Security Forces:''' Awaiting your orders, sir.\\
'''Vince:''' Shoot the bastard down.\\
'''Security Forces:''' Yes, sir.
* EvenEvilHasStandards: In the second game, [[spoiler:Vince]] is disgusted by his employer's lack of honor [[spoiler:trying to evac the site with its personnel left behind when it's clear there's no stopping Kopplethorn]] and actually orders his men to kill him by blowing his helicopter out of the sky. Before he dies, he requests that Snake and Venus let the rest of his men live.
* FightLikeACardPlayer
** In game, though, he often accidentally kills his own men via rocket launcher or thrown barrels.
* FogOfWar: Some levels are in "Search Mode", which functions as this for the player. Even when it's not in effect, though, when the guards aren't on alert, they basically have their own FogOfWar.
* GaidenGame: These games don't have the "Solid" in their titles for a reason: They're totally not in continuity with the MGS titles, despite that they reference them constantly through the cards. Snake is still a legendary solider with a mysterious past, but there's no Liquid, no FOXHOUND, no Big Boss. This allows the creators to give Snake an entirely ''new'' mysterious past to mess with him. In addition, [[spoiler:Snake apparently died in-between AC!D and AC!D2.]]
* GameplayAndStorySegregation: Varies. The first rendered cutscene in the first game shows Teliko in a mission, with her movement and actions controlled by the same cards and grid that the player uses to play, and Roger later tells Snake about the cards, referring to them as, well, cards. But later, cutscenes are more conventional, and in the second game the cards are just referred to as "techniques" that the amnesiac Snake must relearn, as a form of nanotechnological data packets that are released into his brain.
** Lampshaded several times in the sequel - Dalton complains about the CARD system's explanations being confusing and jargon-filled when Wiseman rambles on about them, and while [[spoiler:Lucy/Metal Gear gloats, Snake and Venus calmly discuss Metal Gear's "glitch" that allows you to see its drawn cards.]]
** During the [[spoiler:fight with Venus]], you can [[spoiler:kick her off the ledge]] if you're lucky, allowing you to bypass the entire battle. [[spoiler:She's back up on the railing in the cutscene though.]]
* GottaCatchEmAll: There are a ''lot'' of cards to collect... or buy. Since you only get three cards at a time upon a purchase and they're randomly determined, trying to get all of them is an exercise in ''utter frustration.''
* {{Jerkass}}: Venus to Doc Takiyama... [[spoiler:even after Venus rescues Takiyama from arrest.]]
** [[spoiler:JerkWithAHeartOfGold:]] [[spoiler:Although she was somewhat nicer to her in the latter event.]]
* JigglePhysics: Present in the second game, and very noticeable since everyone's jiggle physics are... activated every time a new scene is rendered. They are ''extremely'' wobbly.
** Fun fact: Snake's web belt has the same boobie physics as Venus' and Takiyama's.
* JigsawPuzzlePlot: The first game manages to be somewhat more confusing than the Solid series. The second game is easy to follow (which is not to say it doesn't get a little ''weird'').
* KudzuPlot: To a lesser degree than in the Solid series, but it does get complicated at times. Lampshaded in 2 - Dalton struggles to keep up.
* LargeHam: The Kerotan who advertises new card packs.
---> '''[---CHRONICLE---] [--CHRONICLE--] [-CUH--]RONICLE!!!! ...PACK!'''
* LongSongShortScene: The main theme from the original ''MGS'' by TAPPY is played all-too-briefly during an advertisement for the [[VideoGame/MetalGearSolid MGS1 Pack]] in ''[=MGA2=]''. The theme has been absent in subsequent Franchise/MetalGear games since then (starting with ''[[VideoGame/MetalGearSolidPortableOps MPO]]'') due accusations of plagiarism, making ''[=MGA2=]'' the only PSP game in the series where it is played.
* LoveMakesYouEvil: [[spoiler:Kopplethorn's entire reason for going batshit insane on his own facility. To some extent, Lucy's motherly drive for her ungodly mutants drives her to use Chaioth to make the world a better place for the test subjects.]]
* LuckyCharmsTitle
* MirrorBoss: The final boss of ''Metal Gear Ac!d [[superscript:2]]'' is [[spoiler:Venus herself]], who draws the exact same cards as you do in your deck, but at an increased rate. You're going to have to play ''very'' cleverly to gain an advantage.
** Or, since she uses EXACTLY the same cards you do, fill your deck with movement and healing cards, position yourself towards the edge of the area facing the entrance next to the pit, get [[spoiler: Venus]] right next to the edge, and punch-punch-kick her off for an easy victory.
* NoFourthWall
-->Snake... er, '''Hans''': Hey, come on! Don't change my name in the text box!
** Snake and Venus calmly discussing the final boss's weakness as it rants on and on about her motivations.
* NostalgiaLevel: The [[spoiler:Metal Gear KODOQUE/Pythagoras]] fight in the second game.
** Somewhat played with in [=MGA2=]'s Arena Mode: The levels are based on [=MGS=], [=MGS2=], and [=MGS3=], but aren't based off any one level. On the other hand, you can fight Liquid Snake, Revolver Ocelot, Vamp, Fortune, The End, The Boss, Venus and Teliko (The first and last can kill you in a single turn using a SOCOM which sometimes hits up to twenty times!). The enemy teams in arena mode always contain two people and are not randomly generated. Sometimes you'll get seemingly strange pairings (like two Telikos) but other times you get the standard pairings (e.g. Revolver Ocelot & Liquid Snake).
* OldSaveBonus
** If you have photo save files from ''VideoGame/MetalGearSolid3: Subsistence'', you can connect your {{PSP}} to your {{PS2}} and view the pictures in [=3D=] using the "Solid Eye" viewer in ''Metal Gear Ac!d 2''. Additionally, you can import a limited number of cards from the first game into your deck in the second game.
** Both Ac!d games also unlock Teliko and Venus in ''VideoGame/MetalGearSolidPortableOps''.
* OnlyMostlyDead: Characters can take a ''lot'' of punishment, even being run over by trains, and still be revived on their own as long as there's another party member still alive.
** Mooks, on the other hand...
* PacifistRun: Much, much harder, especially thanks to the range of tranq weapons, how plentiful lethal weapons are, and CQC's tendency to knock someone off a ledge... with lethal results. Still somewhat encouraged by Dalton, who will mention early on how many people you've killed so far and will note that you ''will'' be killed if you are caught, and [[spoiler:Vince, who later on asks that Snake and Venus spare some of his soldiers]], but beyond that, there's no VideogameCrueltyPunishment, aside from not getting as many bonus points or cards as you do with no kills.
* ParrotExposition: Mostly absent from both games, but lampshaded in the second game.
-->'''Snake''': [[CatchPhrase Metal...]] [[OnceAnEpisode Gear?]]
-->'''Dalton''': Huh? Do you know anything about it?
-->'''Snake''': No... I just felt like I should say that.
* PrivateMilitaryContractors: [=SaintLogic=] Inc. uses mercenaries as guards.
* PsychoPrototype: All of the Test Subjects in the second game, fulfilling the role of QuirkyMinibossSquad. They include a giant cyborg ape who can [[RocketPunch shoot his arms at you]] (and regrow them!), a hypnotist dressed as a traditional Japanese... something... who likes to walk on the ceiling and wielding a grenade launcher, and a man holding a giant fuel container which he drinks out of to breathe fire.
* RidingTheBomb: In the ending for ''Ac!d [[superscript:2]]'', [[spoiler: Snake ends up using the destroyed Metal Gear's missile launching ability to escape by cramming himself into a dummy warhead. It broke every single bone in his body - but he healed, thanks to his status as a super soldier.]]
* SlasherSmile: Harab Serap, the pyromaniac boss of ''AC!D2.'''
* ThemeNaming: The names of the Test Subjects and the Metal Gear itself in ''[=MGA2=]'' all come from the Kabbalah.
* [[ThoseTwoGuys Those Two Guys and that Girl]]: The pilots and Consuela, who accompanied Snake to the US before getting caught. They're reunited at the end, but are absent for most of the game (besides some "encouragement" by Dalton and Snake.)
* ThreeDimensionalEpisode: The second game can be played in 3D, and comes with a "Solid Eye" viewer (a box with separate eyeholes for each half of the screen which shares its name with Old Snake's optical eyepatch from ''[=MGS4=]''). Also used for viewing pictures taken from ''[=MGS3=]'' and ''Subsistence'' in 3D... usually, those of EVA.
* [[spoiler:TomatoInTheMirror: "Snake" from ''Acid 2'' is actually a clone from the original ''Acid'', who supposedly died between the events of both games.]]