[[quoteright:256:http://static.tvtropes.org/pmwiki/pub/images/IO_cover_1052.jpg]]
Also known as ''I/O''. This is the first original game by the company Regista, founded by Takumi Nakazawa of the ''Infinity'' series (''VisualNovel/{{Never7}}'', ''VisualNovel/{{Ever17}}'', and ''VisualNovel/{{Remember11}}'') fame, and a collaboration between Nakazawa and Romeo Tanaka (of ''CrossChannel'', ''KanaLittleSister'' and ''Yume Miru Kusuri'' fame).

''I/O'' takes place in the year 2032. A lunar eclipse occurs in late April, causing a surge of terrorist activities and bizarre incidents. As time goes on, things get worse and worse, with the key to the mystery seemed to be locked away in the MMORPG "Babylon". Four people, through their own ways, investigate the ongoing phenomenon, each one dealing with his or her own problems...

''I/O'', like the ''Infinity'' series, has multiple protagonists, in this case four. Route A features Hinata Aoi, an OrdinaryHighSchoolStudent who has become depressed after the disappearance of his twin sister, Mutsuki. Route B features Ishtar, a young woman in charge of a hacking group known as Criminal. Route C also features Ishtar, but she seems different from herself in Route B. Route D features "HE", a mysterious man who is a legend on the Internet. Once all four of their routes have been completed, they unlock Route E, the true route.

The game was originally released on the PlayStation2 in 2006, but was given an UpdatedRerelease on the PC two years later, with new [=CGs=] and slightly updated artwork (as well as giving sprites to many characters who didn't have any in the original).

A fan-translation was released by [[http://lemnis.ca/ Lemnisca Translations]] on February 22nd, 2014.
----This game provides examples of:
* AFatherToHisMen: [[spoiler:Enlil, who cares deeply for all her followers at Code, who feel a similar loyalty towards her.]]
* AmbiguousGender- Enlil, who is never specifically identified as a man or a woman. [[spoiler:Route E finally confirms that Enlil is a woman.]]
* [[spoiler:AngstySurvivingTwin]] [[spoiler:Mayumi, though notable in that she seems to treat her sister's death on equal level with the rest of her comrades' deaths.]]
* ArcWords- "I'm right here."
** "The beginning is the end, and the end is the beginning."
* ArtificialHuman- [[spoiler:"HE", whose body is 2/3rds mechanical.]]
** [[spoiler:ArtificialLimbs- The true nature of his body- after Hinata's body was severely injured by Enlil, Nergal replaced his legs, left arm, and most of his organs with artificial parts.]]
* AscendToAHigherPlaneOfExistence- [[spoiler:Some of the villains plans. This happens to Criminal at the ends of Routes E and B'.]]
* [[spoiler:AssimilationPlot- Goals of other various villains.]]
* [[spoiler:BackToFront- Routes C, D, C', and D', though played with in that the protagonists of the first two routes are actually experiencing the events in reverse chronological order, but initially believe time to still be moving forward.]]
* {{Badass}}- "HE" lives and breathes this trope.
* BadassLongcoat- Once again, "HE".
* [[spoiler:BadFuture]]- [[spoiler:Routes C and D.]]
* BedmateReveal: Happens in Route D between "HE" and Ishtar, though it's clear that nothing happened between them.
* [[spoiler:{{Bifauxnen}}- Enlil. Mutsuki describes her at one point as having "the body of a woman, but the mind of a man."]]
* BigBad- [[spoiler:While there is no one BigBad in the whole story, virtually all the origins of them can be traced back to eXarch.]]
* BittersweetEnding- [[spoiler:Route E. Mutsuki has finally been saved and everything has finally calmed down, but the rest of Criminal is still dead, Enlil is on the run, Sakuya is nowhere to be found, and Mayumi hasn't shown herself to Hinata in a year.]]
* BolivianArmyCliffhanger: If you haven't cleared the routes required to see their full endings, Route B, C, and D end this way.
* BrilliantButLazy- Hinata is very intelligent and is shown to be able to solve complicated problems in seconds, but he's so apathetic towards life that he's just not interested in it (much to Sakuya's consternation).
* BringMyBrownPants- Andras at a point in Route B.
* BrotherSisterIncest- While nothing explicit happens, there's a lot of subtext between Hinata and Mutsuki, the former who is practically obssessed with the latter. Lampshaded several times, particularly where he questions Yayoi whether or not he has a sister complex.
* ButtMonkey- Kosuke, who seems to exist to get verbally abused by the other members of Criminal.
* ChekhovsGun- [[spoiler:Some seemingly throwaway bits of conversation between Hinata and Yayoi early in Route A turn out to be vital in figuring out the mystery later on.]]
* ChekhovsGunman- [[spoiler:Mayumi, who goes from being a supporting character in Routes A and B to one of the two main protagonists by the end of game.]]
* ClingyJealousGirl: Sakuya for Hinata, which he eventually calls her out on.
* [[spoiler:CloningBlues]]- [[spoiler:Sakuya, who is actually a clone of her former self (also named Sakuya), who was killed in a car accident.]]
* ContemplateOurNavels- The characters go into this occasionally, especially towards the end of Route E.
* CoolShades- "HE" has a pair.
* TheCracker- The cyberterrorist group "Code" is composed of these.
* CrashIntoHello- How Hinata officially meets Masami and Mayumi.
* {{Cyberspace}}
* TheDanza- Kosuke Miyata, who is played by KosukeToriumi.
* [[spoiler:DeadAllAlong- Ea, who was killed prior to the start of the story by Enlil. The "Ea" we see is merely her Shadow Null.]]
* DeadpanSnarker- Hinata, particularly around Yayoi and Sakuya.
** FirstPersonSmartass- The rest of the time.
* [[spoiler:DeathByChildbirth: Sakuya's mother died as a result of this.]]
* DeusExMachina: In-universe example: "HE" is considered to be one of these due to his tendency to appear out of nowhere and solve problems on the Internet.
* DisappearsIntoLight- [[spoiler:Lem, after her death.]]
* [[spoiler:DreamWithinADream: All of Routes A-D and most of E take place in one- in fact, the goal of Route E is to get out of one. It can also be argued, depending on one's interpretation of Route E's ending, that the entire game takes place in one.]]
* DrivenToSuicide- [[spoiler:Isaiah Nagy, who fried his biocomputer after uploading his consciousness online to escape from eXarch's research facilities.]]
* DrowningMySorrows- [[spoiler:Mr. Miyata, Kosuke's father, does this for several months after the death of his son. Notable in that its mentioned he originally quit drinking and smoking after Kosuke was born.]]
* DownerEnding- [[spoiler:Route B, which ends with everyone in Criminal dead or severely injured, while Sakuya's fate is left completely up in the air.]]
** [[spoiler:Route A as well, which ends with Hinata being attacked by Enlil and getting severely injured. It's only the arrival of Yayoi and Cyberforce that prevent him from being killed, but he's still left on the verge of death.]]
* [[spoiler:EasilyForgiven]]- [[spoiler:Enlil, though it's played with that Hinata and Mayumi have to be told by others to let go of their grudges against Enlil first.]]
* [[spoiler:ElectronicEyes- Mayumi, who gets a cyber eye to replace her lost one, which allows her to use Imaginary Vision.]]
* EmergencyTransformation- [[spoiler:Hinata is forced to have 2/3rds of his body replaced with mechanical parts in order to save his life after being severely wounded by Enlil.]]
* {{Expy}}: WordOfGod says that Ishtar and Enlil's costume/character designs were inspired by Julia and Vicious from CowboyBebop, whereas "HE"'s outfit was inspired by Neo's from TheMatrix.
* EyeScream [[spoiler:Mayumi gets her left eye shot out at the end of Route B.]]
* EyesAlwaysShut- Ningirsu.
* ForgottenFallenFriend- [[spoiler:While not exactly fallen, Hinata pretty much never gives Sakuya any thought after he leaves her after Route A, but is eventually subverted in Route A', where his main focus becomes finding her. Averted with Mayumi, as she frequently thinks about death of her comrades.]]
* [[spoiler:FutureBadass]]- [[spoiler:Ishtar in Route C and "HE", who are Mayumi and Hinata, respectively.]]
* [[spoiler:GainaxEnding]]- [[spoiler:Route E'.]]
* [[spoiler:GenderBlenderName- Enlil's real name, Tsukasa, is a name usually given to boys. When Hinata hears her name, he assumes she's a man.]]
* GenkiGirl- Lem, the fairy from Babylon.
* [[spoiler:GirlInABox]]- [[spoiler:This is what happened to Mutsuki.]]
* GogglesDoNothing- Route C Ishtar wears a pair of goggles/sunglasses on her head that she never wears.
* [[spoiler:GreyAndGreyMorality- By the end, it's clear that no single party is completely evil or good, just with different methods.]]
* GuideDangIt- This game is incredibly difficult to clear if you have no idea what you're doing.
** Specifically, there are initially four routes. Sounds simple enough. What the game doesn't tell you is that you have to get a certain other ending to see the full ending of another route. If you don't, then you are left at a "cutoff point", which seems like the ending, but isn't. (In order to see Route A's full ending, you have to clear Route C, Route D must be cleared to see Route B's full ending, Route B needs to be cleared for Route C's ending, and Route A needs to be cleared for Route D's ending). Then you unlock Route E, which, if you clear that, opens up four new routes which must be cleared, and it is then and only then that you can see the True Ending. Sound complicated?
* GunsAkimbo- "HE"'s main style of fighting, with a Desert Eagle in his right hand and a Colt Government in his left. Unlike most examples, however, he usually only focuses on firing one gun at a time.
* GunPorn- One scene in particular in Route D is practically devoted to discussing the specs of "HE"'s weapons in great detail.
* HalfIdenticalTwins: Hinata and Mutsuki have very similar body structures despite being fraternal twins. [[spoiler:As it turns out, they're actually identical twins who were fertilized with a degenerated Y chromosome, which activated for Hinata but not Mutsuki.]]
* [[spoiler:HeelFaceTurn]]- [[spoiler:Enlil.]]
* HeroicSacrifice- [[spoiler:Subverted in Route B': Ningirsu attempts to do this by letting his teammates go ahead while he's stuck next to dynamite, but it's subverted when the sprinklers in the building activate, defusing the dynamite.]]
* HeyItsThatVoice
** Perhaps due to Romeo Tanaka's influence, this game shares several voice actors with Yume Miru Kusuri, such as Michihiko Hagi (who voices Hinata in I/O and Takeshi in YMK), Ryoko Tanaka (who voices Sakuya in I/O and Mizuki in YMK), and Kayo Nagata (who voices Mutsuki in I/O and Nekoko in YMK). Incidentally, the two games were released a month apart from each other.
*** Hinata is also [[FireEmblemTellius Ike]].
* HopeSpot- [[spoiler:At the end of Route B, it looks like Criminal and the others are finally going to make it to the helicopter pad... and then Hinata's father gets shot. [[DownerEnding It all goes downhill from there.]]]]
* IWantMyBelovedToBeHappy- [[spoiler:Mayumi acknowledges that while she loves Hinata, he has Sakuya, and so she resolves to let them be happy together. However, in the ending of Route B', where Hinata is unable to save Sakuya, it's implied that Mayumi will eventually confess to Hinata.]]
* InferioritySuperiorityComplex- [[spoiler:Masami, who's haughty attitude is used to hide the fact that she feels inferior to her sister Mayumi, who is more loved by the rest of the members of Criminal. Ironically, Route C' reveals that Mayumi also felt inferior to Masami.]]
* ItsNotYouItsMyEnemies- [[spoiler:Hinata uses a variation of this to leave Sakuya.]]
* [[spoiler:ItsPersonal]]- [[spoiler:It's eventually revealed that many members of "Code" are actually normal people who had relatives who worked at eXarch that got killed off by them, hence why their main target is eXarch.]]
* JapaneseRanguage- Actually used as a plot point once. [[spoiler:When trying to solve Mutsuki's crossword puzzle, Hinata can't figure out the clue for the word "Lunar Excursion Module" due to the lack of an L in Japanese, and so is stumped until Mayumi helps him. She even lampshades the trope.]]
* [[spoiler:JourneyToTheCenterOfTheMind]]- [[spoiler:Hinata and Mayumi have to do this to awaken Mutsuki by diving into her brain in the Enigma system. However, this gets them trapped into an infinite loop by "Marduk".]]
* [[spoiler:KillEmAll]]- [[spoiler:The ending of Route B.]]
** [[spoiler:Several bad endings also count.]]
* [[spoiler:LegacyCharacter]]- [[spoiler:Ishtar becomes this, with Mayumi taking up the role of "Ishtar" after Route B.]]
* [[spoiler:LongLostRelative]]- [[spoiler:Ishtar's birth mother is Shinobu Shinozuka, making Mayumi and Masami her sisters. However, none of them know this, nor do they ever learn it.]]
* LoveEpiphany: [[spoiler:Mayumi has one in Route B', where she realizes she loves Hinata.]]
* LoveTriangle: [[spoiler:Masami likes Kosuke, who likes Mayumi (who is completely clueless to Kosuke's affections.]]
* ManipulativeBastard: [[spoiler:Nabu/Mr. Kawahara.]]
* MatrixRainingCode: Heavily featured in the OP. In the original PS2 version, it was colored green, but in the PC version, it was recolored blue.
* MayDecemberRomance- [[spoiler:Mutsuki fell in love with Enlil when she was only ''12 years old''.]]
* MaybeEverAfter- [[spoiler:Route B' ends with slight hinting at Hinata and Mayumi possibly getting together in the future, but nothing explicit happens.]]
* MeaningfulEcho- [[spoiler:When Hinata and Mayumi meet for the first time, Mayumi's fallen to the ground via a CrashIntoHello, and Hinata reaches his hand out to her (which she responds to with bewilderment). In the ending of Route B', when Hinata is sitting on the ground, depressed about his failure to rescue Sakuya and the permanent loss of his father and "HE", Mayumi comes over to him and reaches her hand out to him in the same way, with him giving a bewildered stare.]]
* MeaningfulName- Several, some of them Lampshaded in-story, while others come from WordOfGod:
** When reversed, the kanji in Hinata Aoi's name forms the kanji for "sunflower".
** The kanji in Mutsuki's name literally mean "dreaming moon".
** If you take out the middle kanji in both Mayumi and Masami's names, you get the kanji for "truth". This is explicitly pointed out by Ishtar at one point.
** [[spoiler:Isaiah Nagy (the first IC and the source of "HE") and Nami Izawa (Ereshkigal) both have similar roots: Reading the first part of Isaiah's first name and last name yields "Izanagi", while reading Nami's name in the Japanese order yields "Izanami", the god and goddess of creation in Japanese mythology.]]
* MesopotamianMythology- A major theme in the game, with characters, locations, and terms being named after Babylonian and Assyrian mythological terms.
* MindScrew: Especially in the game's second half.
* MysticalPregnancy: In-universe, it's been observed that in recent years, there have been rare cases where girls have been conceiving children on their own without need of a sperm, known as virgin births. [[spoiler:Both Ishtar and Sakuya are the result of these.]]
* NotSoDifferent- [[spoiler:It turns out that both eXarch and Code have the same goal- to create a future for Imaginary Children. It just so happens that eXarch wants to use them for their own benefit.]]
* [[spoiler:NiceJobBreakingItHero: When "HE" is summoned by Mayumi in order to save Criminal at the end of Route B, he seals the shutters and pours gas into the room to kill the remaining members of Code... only to end up accidentally killing Ishtar and Masami in the process.]]
* ObliviousToLove- Hinata seems completely unaware of Sakuya's rather obvious feelings for him. [[spoiler:Route A' subverts this, where it reveals he ''was'' aware of her feelings, he just didn't feel like he could accept them in his mental state at the time.]]
** A rare inversion occurs as well: [[spoiler:Mayumi is actually in love with Hinata, but doesn't think her feelings for him are love at all, thinking it just to be mere concern on her part. She eventually has a LoveEpiphany in Route B'.]]
* OneSteveLimit: Subverted, [[spoiler:there are two "Sin"s in Babylon: the original one, Mutsuki, and the new Sin, who is a drug dealer and a member of "Code" (it's eventually revealed that "Sin" is a title given to the commanding officer of [=DlroW=]). The latter ends gets killed by Enlil in the real world for disgracing Mutsuki's name and for abusing his power. Furthermore, Andras becomes the next Sin.]]
* PetTheDog- The Drama CD "Damsel Overbrowse" is this for Masami, [[spoiler:who, being the only one who learns that Mayumi's best friend Kaori has committed suicide, pretends to be Kaori for Mayumi rather than break the truth to her.]]
* PietaPlagiarism: "HE" at the start of Route C and D, holding [[spoiler:Mutsuki's body]] in his arms.
* [[spoiler:PoweredByAForsakenChild]]- [[spoiler:The Enigma system, which is powered by Mutsuki by putting her a sleep that lets her brainwaves control the system.]]
* PlayfulHacker- Many, but especially Ishtar, who one time manages to make a program to delete viruses like a shooting mini-game.
* [[spoiler:PoorCommunicationKills: A good deal of the plot could have been avoided if Enlil had just put her pride aside and had been willing to listen to others.]]
* PutOnABus- After the end of Route A [[spoiler:and her brief appearance in Route B, Sakuya]] just disappears from the plot altogether. [[spoiler:She eventually reappears in Route A'.]]
* RapunzelHair- Ishtar. [[spoiler:Ereshkigal's hair is even longer.]]
* RefusalOfTheCall- [[spoiler:In one of Route B's Bad Endings, Ishtar refuses to stop "Code"'s assault on eXarch, and instead has Criminal run away to a remote island. Unfortunately for them, TheCallKnowsWhereYouLive...]]
* RoboticReveal- [[spoiler:"HE" in Route C is revealed to have a mechanical left arm.]]
* SacrificialLion- [[spoiler:Mika in Route B.]]
* ShrinkingViolet- Mayumi.
* ShoutOut- Several to various pieces of literature, such as Literature/AliceInWonderland, The Once and Future King, Theatre/{{Macbeth}}, Literature/TheEpicOfGilgamesh, Literature/AConnecticutYankeeInKingArthursCourt, [[Film/BladeRunner Do Androids Dream Of Electric Sheep]]... the list goes on.
* [[spoiler:SoleSurvivor]]- [[spoiler:Mayumi is the only member of Criminal who survived the assault on eXarch.]]
* SoundtrackDissonance- [[http://www.youtube.com/watch?v=D6ZOBInkJL8&feature=related "Subnetwork"]], a rather calm, soothing theme... which often plays as something bad is happening or when people are dying.
* StalkerWithACrush- Andras to Mika, which traumatizes her horribly. [[spoiler:In one of the Bad Endings, he ends up killing her, while in another, she shoots him dead.]]
* SunglassesAtNight- "HE" always wears sunglasses, regardless of time or place. [[spoiler:When he does take them off, it's to signify that he's fully speaking as Hinata at that moment.]] Justified in that they're meant to protect his eyes from flash grenades.
* [[spoiler:SuperPoweredAlterEgo]]- [[spoiler:This is essentially the relationship between Hinata and "HE": after "HE" is downloaded into Hinata, he can use his body to fight, but both Hinata and "HE" are existing at the same time, and don't require to "shift" or anything. However, Hinata has none of "HE"'s abilities.]]
* SurprisinglyGoodEnglish- With a few exceptions, the written English in the game is correct, both grammatically and spelling-wise.
* [[spoiler:SurvivorsGuilt]]- [[spoiler:Mayumi. The scene where it hits her is even called "Survivor's Syndrome".]]
* TalkingToHimself- Twins Mayumi and Masami Shinozuka, who share the same voice actress. [[spoiler:"HE" shares a voice with Hinata, "Ashur", and Isaiah, while Lem shares a voice with Mutsuki "Marduk", and Ereshikigal.]] There are also several other characters in the game who share voice actors, but that's more due to the limited voice cast rather than other reasons.
* TakeUpMySword- [[spoiler:Ishtar to Mayumi at the end of Route B, giving her the rosary that her "Onee-sama" gave to her, as a sign that her role has ended.]]
* TakingTheBullet- [[spoiler:Kosuke dies in Route B after taking a bullet aimed at Mayumi and Masami.]]
** [[spoiler:In Route E, Lem does this for Mayumi (with Ishtar's appearance), taking a bullet aimed for her.]]
* CoolBigSis/ TeamMom- Ishtar for the rest of Criminal.
** [[spoiler:As it turns out, she literally is the former for Mayumi and Masami.]]
* TechnicalPacifist- [[spoiler:During the raid on eXarch in Route B, Kosuke notably selects a tranquilizer gun as his weapon.]]
* ThemeNaming- Many characters, locations, and terms are named after entities from Babylonian/Assyrian mythology, and there are many allusions to said mythologies.
* ThirdPersonPerson- Mayumi.
* TomatoInTheMirror- [[spoiler:Ishtar in Route C is actually Mayumi, who took up Ishtar's role after Route B.]]
** [[spoiler:Also done in Route D, where it reveals that "HE" is actually Hinata, but as this is revealed in Route C and is heavily hinted in the route, it's not as much as a surprise.]]
* TookALevelInBadass- [[spoiler:Hinata and Mayumi, but especially Hinata, who squares multiple levels at once when he becomes "HE".]]
* TragicKeepsake- [[spoiler:After his father's murder, Hinata wears the chronograph that his father once wore.]]
* {{Tsundere}}- Sakuya for Hinata and Masami for Kosuke.
* TwinSwitch- In an additional scene in Route B, [[spoiler:Masami attempts to pretend to be Mayumi in order to confess to Kosuke, since she thinks Kosuke likes Mayumi more. However, Kosuke quickly sees right through her, and tells Masami that though he likes her, he doesn't like her in that way.]]
* [[spoiler:VictoriousChildhoodFriend]]- [[spoiler:Sakuya in Route A'.]]
* VirtualGhost- Shadow Nulls, who are essentially the remainders of someone who used {{Cyberspace}} frequently, and can continue to live after death in that world, but cannot interact with the real world unless they have someone living to do it through.
* WhatHappenedToTheMouse: In-universe example: [[spoiler:Hinata wondering what happened to Sakuya is what drives the plot of Route A'.]]
* [[spoiler:WhereAreTheyNowEpilogue]]- [[spoiler:Route E's epilogue shows Hinata, Mayumi, and Mutsuki one year after the end of the game.]]
* {{Yandere}}- [[spoiler:Enlil, who's ''very'' possessive of Mutsuki.]]
* YourMindMakesItReal- Specifically averted, though people with biocomputers can have their biocomputers potentially short, or at the very worst, fry their brain.
----