->''"Rule 20: Don't eat mermaid flesh. Period."''
->-- '''Livin' and Lovin' In The Anime Universe: A Basic Guide.'''

It is said that eating the flesh of a mermaid can give one eternal life. Yuuta, a poor fisherman, didn't really believe the tales, but when the rest of his friends tried some, he ate some as well. Less well-known is that mermaid's flesh is fatal to most, and almost all those who don't die are transformed into hideous mindless immortal beasts called Lost Souls. Within days, Yuuta was the sole survivor of his friends. 500 years later, he is physically no older, but has grown weary of his lonely existence, and seeks the one creature that might be able to restore his mortality -- a mermaid.

In the first of the linked stories, he meets a new immortal (the first he's met in his life) -- a beautiful 15-year-old named Mana, who's been raised in utter seclusion from childhood while being groomed for immortality. The two travel together, in search of a cure that may not even exist -- after all, what else is there for them to do? It's not as if they don't have all the time in the world. The other tales are divided among Yuuta's reminiscences of his past adventures over the centuries, and their new adventures in their quest for mermaids.

The manga was the creation of Creator/RumikoTakahashi, famous for creating ''Manga/RanmaOneHalf'' and ''Manga/InuYasha,'' but is far darker in tone. No over-the-top comic antics, but the terrible things that happen when humans try and seek immortality. Two {{OVA}}s were made of individual stories back in the 80s -- ''Mermaid's Forest'' and ''Mermaid's Scar''. The former name was used as the title for a recent TV series which covered almost all the stories (with the exception of ''Mermaid's Gaze''; the ''Mermaid's Scar'' episodes were not broadcast, and were on the video release only). Most of the stories took 2 episodes.

It's important to note that these are Japanese mermaids, with fangs, fish-eyes and an appetite for human flesh, rather than the siren-like western version. The mermaids are also immortal and (in the manga version at least) can likewise only be killed by beheading or poison. Some can take on human form, but this does slowly age, and they need to feed on an immortal woman to take on a new human form (that of the woman they feed on).

!!This show provides examples of:

* AppliedPhlebotinum: Mermaid's flesh, blood, ashes and liver are all used at some point. Also, a magical Buddhist ritual called ''hangon'' is used in one story.
* AndIMustScream: While most Lost Souls are mindless beasts, [[TragicMonster at least one of them still had a conscious mind]].
* BlessedWithSuck: Immortality is pretty horrible (not immune to pain, outliving everyone you know, having to move constantly), even without having to deal with [[spoiler:Masato forcing you to be his replacement Mommy]]. Also, due to the reveal that [[TheCorruption the rate of conversion]] is different for everyone, there's no guarantee that an immortal won't become a Lost Soul at some point in the distant future.
* BodyHorror: [[TheCorruption The process]] by which a victim is transformed into the hideous, hulking abomination known as "Lost Soul." Sometimes the process is incomplete, leaving only part of the victim's body changed.
* CainAndAbel: From ''Mermaid's Gaze'', Akiko is Abel, Shingo is Cain. Played with in ''Mermaid's Forest'' in regards to Sawa and Towa, [[spoiler: as it turns out Sawa had used Towa as a guinea pig to test the mermaid's blood before she used it on herself, and this led to Towa's embitterment.]]
* CanineCompanion: Shiro to Towa. The reason they are so close is because Towa feels like Shiro is the only one who understands her, as they are both deformed due to the effects of the mermaid's blood.
* CelibateHero: Yuta has actively shied away from any romantic relations after his first marriage due to his immortality. Too bad that he also seems to be a bit of a...
* ChickMagnet: Not played for any laughs. In fact the story behind ''Mermaid's Promise'' was that [[spoiler: Nae was murdered by her jealous fiance when he realized that she was in love with Yuuta instead of him]].
* ClingyJealousGirl: Mana's actions around Yuuta when he is with another girl seem to come off as jealousy, which she herself appears to be unaware of.
* CreepyDoll: There appears to be a life-sized one of a girl Yuuta used to know. [[spoiler:Turns out, it's the ''actual body of that girl''. Which, thanks to the mermaid's flesh she used for poison on herself, hasn't rotted at all since the day she died]].
** [[spoiler: That's assuming she did die - there's a suggestion that she may have actually been [[AndIMustScream paralyzed and fully conscious.]]]]
* DarkerAndEdgier: Than [[Manga/UruseiYatsura most of]] [[Manga/RanmaOneHalf Takahashi's]] [[Manga/MaisonIkkoku other works]], at least.
* TheDitz: Mana displays evidence of this at times due to her extremely sheltered upbringing, not knowing what cats are, how to make tea or the first thing about romance -- the last of which adds an interesting element to her relationship with Yuuta.
* DrivenToSuicide: In ''Mermaid's Gaze'', Akiko gave her brother and herself mermaid's flesh as a poison in order to kill themselves, although Shingo didn't know this. Akiko did so because Shingo was an uncontrollable sociopath who often relied on Akiko to get him out of trouble, until Shingo killed her finance. [[spoiler: It didn't work though, as Shingo became immortal and Akiko's corpse was perfectly preserved, though it's implied that she's fully conscious yet locked in her own body.]]
* EnfantTerrible: [[spoiler: Masato]]
* EvilAllAlong: [[spoiler: Sawa Kannagi and Masato]].
* EyeRemember: In ''Mermaid's Gaze''.
* FailureIsTheOnlyOption: The first story claims that there may be no cure after all -- but Yuuta doesn't see any better options than to keep searching.
* FlyingDutchman: Yuuta can't stay in any one place for too long for fear of his eternal youth causing resentment.
* GoodThingYouCanHeal
* HeroWithBadPublicity: [[spoiler: Misa. At first it looks like she's an AbusiveParent trying to kill her son Masato. Then it's revealed Masato is the one who gave her the Mermaid's Flesh and has been doing the same thing to countless women for 800 years. She's actually been trying to find out where he's been keeping his supply of Mermaid's Flesh to stop him from ruining the lives of other women.]]
* IdiosyncraticEpisodeNaming: Most (though not all) of the episodes are named ''Mermaid's <noun>''.
* ImAHumanitarian: Not only do mermaids and humans eat each other's flesh, but mermaids in human form need to feed on the flesh of their own kind when pregnant before they can give birth.
* ImmortalLifeIsCheap: Our main immortal characters are served some nasty treatment (as in "Hey, I need your [[spoiler:liver]], so I'll just take it out while you're still alive and conscious") if not outright death a couple of times.
* IsItSomethingYouEat: When Yuuta asks Mana if she is jealous, she is unfamiliar with the concept and thinks he was asking if she wanted Jello and asks if it tastes good.
* LockedIntoStrangeness: Towa Kannagi.
* LongestPregnancyEver: Isago is after mermaid flesh for the unborn child of her previous husband. [[spoiler: [[LampshadeHanging This confuses the chief, as her husband's been dead for quite awhile.]] She's actually been pregnant for three years, [[OurMermaidsAreDifferent due to her being a mermaid]]]].
* LoveDodecahedron: Averted, unlike most of Takahashi's other works. Yuuta had been married once when he first ate the mermaid's flesh, and over the span of his lifetime he's met other women who've fallen for him, yet none of this has had an affect on his standing with Mana since they were all dead before he met her. And even then, it's not clear as to whether or not Mana's feelings are platonic or not.
* MadWomanInTheAttic: [[spoiler:Towa's]] main grief with [[spoiler:Sawa]] is that the latter locked her away to prevent other people from seeing her, while [[spoiler:Sawa herself]] lived a full and happy life with a spouse and children.
* MamaBear: Deconstructed via LoveMakesYouEvil in ''Mermaid's Mask''.
* MayflyDecemberRomance: Yuuta's marriage 500 years ago became poisoned by his wife's resentment of his eternal youth. Since then he's refused to become seriously involved with mortals, although there have been several women interested in him.
* MillionToOneChance: Unlike most, the chances of becoming immortal from eating mermaid flesh seems to ''genuinely'' be one in a million.
* NighInvulnerability: Regeneration variety -- all immortals and Lost Souls can be killed only by beheading, burning down to ''ash'', or a poison made by rotting mermaid flesh in certain herbs. Everything else they just heal. Missing limbs and the like can also be replaced by [[AppendageAssimilation grafting a replacement on]], which leads to some particularly gruesome scenes...
* NoNameGiven: Masato's [[spoiler: current]] mother and the old woman from ''Mermaid's Gaze'' weren't given names. In the ''Mermaid's Scar'' OVA, Masato's mother was called Misa.
* OffWithHisHead: The traditional way of disposing of immortals and Lost Souls. Others get more creative.
* OlderThanTheyLook: Naturally.
** Special mention may go to Towa, who only looks young on the outside. Her internal organs are those of an old woman.
** Mana is physically 15, but probably around 30 by the end of the series.
* OurMermaidsAreDifferent: ''Very'' different.
* RazorFloss: Shows up in ''Mermaid's Scar''.
* ReallySevenHundredYearsOld: Probably about half the characters, thanks to the premise. Yuuta is about 500 but looking about eighteen.
* ReplacementGoldfish / ArtificialHuman: "The Bone Princess."
* SelfMutilationDemonstration: Yuta does this several times, to the point that he is ''told'' to do it to prove his immortality or he [[LampshadeHanging lampshades]] it when Mana hands him a pair of scissors to give another demonstration.
* SkinshipGrope: Takes on a chillingly sinister tone in ''Mermaid's Forest''.
* ToBecomeHuman: The main goal of the recurring characters.
* TheVirus: "Lost Souls."
* WalkingTheEarth
* WhatIsThisThingYouCallLove: When Yuuta outright asks Mana if she's in love with him, she answers she doesn't know what being "in love" means.
* WholeEpisodeFlashback: "The Village of Flying Fish" and "The Bone Princess."
* WhoWantsToLiveForever: Played ''very'' straight. [[spoiler: Subverted, possibly, by the ''Mermaids Do Not Smile'' story; Yuuta discovers the cure really was just a myth, and is then very nearly KilledOffForReal in a rather gruesome fashion--the final scene seems to imply that after that harrowing experience, death no longer holds the same appeal for him]].