''Gyo'' is a {{Seinen}} horror manga by Creator/JunjiIto, also known for creating ''Manga/{{Uzumaki}}'' and ''Manga/TheEnigmaOfAmigaraFault'' (which, incidentally, is an extra story in the back of the second volume).

During World War II, the Imperial Japanese Army commissioned a secret experiment to create biological weapons. The end result - a deadly bacteria that infected corpses and released nauseous gases, and a walking mechanism fueled by these gases, for the purpose of spreading infection. This plan was averted when the ship carrying the bio-weapon was sunk by American bombers during the war - but the bio-weapon remains active, infecting the sea life and spreading back into Japan in the form of what appears to be [[NinjaPirateZombieRobot undead walking fish.]]

It gets worse.

An animated OVA for the story was released on February 15, 2012 by Ufotable. There are some [[AdaptationDistillation artistic liberties]] with the OVA story, such as Kaori becoming the protagonist, as well as new side characters to boot.

Trailer for the OVA can be viewed [[http://www.youtube.com/watch?v=Ur_WqzyJj_U here.]]
!!''Gyo'' contains examples of:

* AndIMustScream
* AssShove: One of the rare examples played entirely for horror, without a single hint of humor. [[spoiler:[[http://i5.mangareader.net/gyo/13/gyo-2375855.jpg It happens when infected humans get wired into the walking machines.]]]]
* BodyHorror: Those infected with the gas become bloated and covered in boils. A particularly gruesome moment as a result of the walkers comes when [[spoiler: Yoshiyama tries to remove the tubes from Kaori's mouth. Her lips instead stretch when the tubes are pulled, and her mouth is visibly irritated once Yoshiyana gives up, making it clear that these machines integrate and become irremovable.]]
* BodyOfBodies: Something like this becomes a commonplace sight, [[spoiler: with larger fishes' walkers taking on many human bodies to power them.]]
* BrickJoke: The first walking fish is smashed and bagged, [[spoiler: but its bacteria keeps producing enough gases to make it ''float to the mainland.'']]
* CassandraTruth: The fact that Kaori even goes into full-blown fits over a scent only she can smell didn't help.
* CircusOfFear: The setting of the penultimate chapter. Yes, there are clowns.
* CosmicHorrorStory: The circus master's interpretation of the gas is that it is coming from (or at least controlled by beings from) another world, which would definitely put the story in this genre. However, there are also other explanations offered up for the gas, of considerably more mundane origin, and it's never made clear who's right.
* CreepyUncle: Tadashi's uncle, also a MadScientist.
* DepopulationBomb
* DevouredByTheHorde: Tadashi nearly becomes a victim of this when he lands on a huge group of undead tiny fishes but he was eventually saved off-screen.
* DownerEnding: [[spoiler:Apart from the hero meeting up with LaResistance, it's a total downer.]]
** [[spoiler: The OVA tries to make the ending [[FromBadToWorse even worse]] by showing scenes of the infection in different countries, a clear sign that unless an immunity is developed, most of humanity is pretty much screwed.]]
** Unlike the manga, the OVA implies [[spoiler:that there is an organized effort by the Japanese government (or its remnant) to evacuate the survivors from Japan, or at least from Tokyo. Similar evacuation efforts may very well be taking place in other countries. Also, the internet is shown to be still functional when Tsuyoshi uploads the plague data.]]
* EditorialSynaesthesia: Stink lines.
* {{Everybodys Dead Dave}}: [[spoiler: Tadashi is the only named character we know for sure survives.]]
** In the OVA, [[spoiler:Kaori is this.]]
* EvilHand: One of the legged machines is removed from the fish it was fused to, after which it latches onto a scientist's arm. Though he promptly [[LifeOrLimbDecision amputates it]] ''it starts crawling around for a while.''
* EvilSmellsBad: Basically what the story is about.
* {{Evil Uncle}}: More like callous and really crazy uncle.
* {{Fanservice}}: Though it somewhat serves a purpose to the plot (washing the smell away), Kaori's shower scene in the first chapter is one of Junji Ito's very, very rare examples (possibly the only example) of nudity that seems partially meant to be eye-candy and not just horrifying. The shower scene is also used to implied that the zombie fish is around, though, but that isn't too scary, at least not more scary than other scenes. This unique case of fanservice is probably due to the quite light-hearted start of the manga. The OVA also has its share of the shower scene as well.
** FanDisservice: [[spoiler: When the Death Stench starts to infect humans some chapters later, there is again a lot of nudity, also of Kaori, but this time, it certainly isn't played for fanservice. The term {{Squick}} only scratches the surface...]]
* FateWorseThanDeath: Ignoring the physical BodyHorror for a bit, the bacteria slowly paralyses the victims' muscular and nervous systems, keeping them vegetative but very much alive. [[spoiler: Kaori is even conscious enough to reactivate her walker and try to MurderTheHypotenuse.]]
* FogOfDoom: Envelopes the setting pretty quickly.
* ForScience: The apparent motivation for [[spoiler:Tadashi's uncle improving upon the design of the old, rusted mechanical legs and constructing the two prototype walkers. What exactly he planned on doing with them is a mystery for the ages.]]
* {{Ghostapo}}: A variation; the gas is implied to be somehow related to the [[https://en.wikipedia.org/wiki/Unit_731 horrific biological warfare experiments]] conducted by Japan during World War II, and possibly controlled by the spirits of people killed by them. This, oddly enough, makes ''Gyo'' one of the few works of Japanese fiction to address Japan's wartime atrocities.
* {{Hot Scientist}}: Ms. Yoshiyama.
* {{Just Before the End}}
* LaResistance: In the form of a group of immune students from Kyoto University, who are studying the disease with the intention to create a vaccine.
* KarmicDeath: The soldier who shoots at [[spoiler: the legged machines driven by human bodies, and then gets impaled by one.]]
** In a sense, the karma goes even deeper than that, as the Imperial Japanese Army - the predecessor of the JSDF - was the organization that created the bio-weapon in the first place.
* MadScientist: Tadashi's uncle.
* MeaningfulName: "Kaori" comes from Japanese words referring to smell, and she has a very powerful sense of smell (causing her to pick up the Death Stench before anyone else).
* MaybeMagicMaybeMundane: What compels the walkers' behavior. They straddle the line between being a sentient bio-weapon and being literally powered by ghosts. The story provides no definitive answer.
* NoBiochemicalBarriers: The disease can apparently infect ''anything''. It was originally developed to infect mammals, specifically the original plan was to release infected dogs and have them spread it to enemy soldiers, but in addition to this we also see such diverse things as elephants, cattle, pigs and whales, the eponymous fish and sharks and even ''squid''. Then again it is supernatural bacteria.
* NinjaPirateZombieRobot: Zombie fish. With robot legs. [[spoiler: Actually, just robot legs. Made by ghosts]].
* RaisingTheSteaks
* RealityEnsues / SquareCubeLaw: At one point, a sperm whale on robot legs crawls out of the sea... but immediately crushes the legs under its weight.
* SceneryGorn: The final chapter plays this to the extreme, with giant walkers composed from mountains of dead bodies.
* ScreamingWoman: Kaori. [[{{Narm}} In nearly every scene]]. In the anime, however, she is noticeably much calmer.
* SpiderTank: All the walking fish are fused with these. They steadily get bigger to carry bigger bodies.
* {{Threatening Shark}}s: Walking ones, with spider legs. An early terror.
* TimeSkip: Invoked when the hero falls into a canal and goes into a ConvenientComa for a month.
* ZombieApocalypse: The ending.

!!!Tropes from the OVA:
* AdaptationDistillation: The OVA adds a few more characters to the roster, while [[DemotedToExtra downplaying]] Tadashi and his uncle and making Kaori the main character. Given that the original manga only had four main characters and was relatively short and straightforward, it's natural to assume that more characters would be added to give the OVA more meat to the story.
** Kaori herself has parts of her personality removed, specifically her obsessive clinging to Tadashi and her overly sensitive nose, while maintaining the same devotion to Tadashi. Her more negative attributes are shifted to two new characters, Erika and Aki.
** Tadashi retains his levelheaded personality even if his role is downplayed in Kaori's favor. [[spoiler: Their roles are generally switched, as Tadashi is the one who becomes infected by the gas while Kaori is trying to find him. The two don't even spend any screen time in the same room up until Kaori finally finds him at his uncle's house.]]
*** [[spoiler:Right before he is attacked by a shark]], Tadashi warns Kaori over the phone not to come to Tokyo, as the walking fish plague has spread there. Not that staying in Okinawa would have been any better...
* AlienSky: A grey, swirling sky is shown off throughout the OVA. [[http://i.imgur.com/RiDIz.jpg It's actually pretty by the end.]]
* AThreesomeIsManly: Erika and her two strangers.
* BlandNameProduct: In the OVA, Tsuyoshi uses a [=BloodBerry=].
* BloodierAndGorier: The manga had very little blood; the OVA has ''gallons''.
* BloodSplatteredInnocents: At one point in the OVA, Kaori slips and falls in a large puddle of blood.
* TheDogBitesBack: Aki to Erika [[spoiler:once she's been infected.]]
* FinalGirl: [[spoiler:In the anime, where Kaori is the only named survivor.]]
* GivingThemTheStrip: Kaori sheds her sneaker to escape when she steps on a vacant set of walking-legs.
* HeavyVoice: This, combined with VoiceOfTheLegion, is a side effect of [[spoiler:Erika's]] infection.
* MadnessMantra: Aki chants "Die, die, die, die" under her breath as she simmers over Kaori leaving and Erika engaging in a threesome.
* MonumentalDamage: The world's doom is sealed with a montage of swarms at the Eiffel Tower, Taj Mahal, Empire State Building, and Kremlin.
* {{Spared by the Adaptation}}: [[spoiler:Kaori]] in the anime.
* UnreliableNarrator: Erika and her gentleman friends are less than truthful in their television interviews.
* VoiceOfTheLegion: This, combined with HeavyVoice, is a side effect of [[spoiler:Erika's]] infection.
* WolverinePublicity: There's quite a few of those landsharks crawling around in the OVA.