Taiyo Nakabaru is on a school trip to Okinawa. While he is there, he wants to go SCUBA diving off the waters of Zamimi Island, the place where his father disappeared 10 years previously. He gets off the plane at the airport and tries to help a girl with some luggage. A conversation about SCUBA Diving ensues, but when Taiyo lets slip that he wants to see the ocean that his father disappeared in 10 years ago, the girl slaps him and blames him for her mother's death. Taiyo doesn't remember anything about it. Taiyo is then introduced to the Miyara sisters: The oldest is Nagisa Miyara, a diving instructor and the owner of Blue Wave Diving rental shop. Shibuki Miyara, the girl who slapped him, is around Taiyo's age. Finally, there is Mio Miyara, a stoic girl who is younger than the rest of the cast.
However, the vacation Taiyo anticipated is quickly interrupted. He soon witnesses a strange symbol shining from the water, and immediately afterwards in shoved off a cliff, landing in the sea. In it, he sees a white humpback whale. That is not all. Because Taiyo is broke and SCUBA Diving is expensive, he winds up working for Blue Waving Diving Service. Over time, more strange incidents involving the symbol occur, the plot thickens, and more character appear. Motivations and allegiances remain uncertain, characters find themselves at cross-purposes, and the mystery of the symbol becomes ever more pressing.
The series is a mixture of Romance, Slice of Life, Mystery, and Harem. The beginning of the story is rather slow, focusing more on Taiyo's experiences with first coming to Okinawa and the daily activities in running a dive shop.
The series can be compared to Umi no Misaki in that they are both romance series set on fictional Okinawan islands. They both have a dreamy feel to them as well as feature character development and mystery. However, Umi no Misaki has a dreamy feeling that could be described as pleasantly relaxing. IO has a sense of supernatural, unreal urgency about it that makes it a rather gripping read once the plot gets going.
This manga provides examples of:
- Above the Influence: Mio makes a number of attempts to seduce Taiyo into having sex with her, and while he steadfastly turns her down he initially wonders how long his resolve to do so can last.
- Abusive Precursors: While whether the ruins, symbols, and lights were created by people or aliens, is never explored, it still doesn't get much more abusive than forcing future generations to practice ritual murder.
- Action Survivor: Taiyo becomes this. He's fallen from a high cliff multiple times, survived Nitrogen poisoning, Oxygen toxicity, being attacked by a mercenary, rescuing several people from drowning, massive underwater explosions, being captured by guerillas, evaded armies, and other attempts on his life.
- Advanced Ancient Acropolis: Several. Attempting to explore one of these almost proves fatal for Taiyo and Shibuki. It's eventually revealed they belonged to a prehistoric whale-worshipping culture, which Taiyo is descended from.
- All Women Are Lustful: Mio repeatedly tries to seduce Taiyo, and Mar succeeds in having sex with him after drugging him with coca tea.
- An Arm and a Leg: Subverted by Henri Hans; he hasn't lost his limbs so much as become paralyzed without visible reason.
- Animal Wrongs Group: Nirvana is seemingly an environmental activist group turned terrorist organization. They're revealed to be a sector of the US military opposed to the effort to activate the ruins.
- Animal Nemesis: The white whale to a surprising number of people.
- Animal Motifs: Whales show up a lot.
- Art Evolution: The earlier character designs feature more heavily angular lines which have the unfortunate effect of often making characters look peeved, or giving them a rape face.
- As Long as It Sounds Foreign: Lago, a quarter-Japanese girl living in Peru. Averted for most other Europeans, who have sensible European names, including Lago's sister Mar.
- Author Appeal: Loads of gratuitous scenes of girls in bikinis, as well as more wet t-shirts than scientifically plausible.
- Babies Ever After: Taiyo and Shibuki have a son named Kai in the epilogue.
- Big Brother Instinct: Causes a good deal of drama late in the manga involving Mio, who has romantic feelings for Taiyo.
- Bittersweet Ending: The mystery is solved and the cycle is broken, but the island is mostly destroyed by a landslide caused by a nuke left at the bottom of the ocean.
- Boxing Lessons for Superman: The diving equipment never changes, but the skill and creativity of the characters continually pushes the boundaries of what they can do with it.
- Breaking the Fellowship: The Peru arc splits up the group, leaving Shibuki and Nagisa in Japan while Taiyo and Mio travel to Peru.
- Break the Cutie: This series hates cute kids, especially younger siblings.
- Broke Episode: Quite a number of them actually, and financial stresses frequently drive the decisions that the characters make.
- Brother–Sister Incest: Narrowly averted between Taiyo and Mio, who has romantic feelings for him and repeatedly expresses her intent to have his children, but is revealed to be his half-sister in the second-to-last chapter and settles for a platonic relationship with him in the epilogue.
- But Not Too Foreign: All of Taiyo's love interests turn out to be at least a quarter Japanese.
- Call-Back: Several, expecially common when the symbol is concerned.
- Cerebus Syndrome: For a manga that started out being about SCUBA Diving and flirting with pretty girls, it got pretty damn dark.
- Chaste Hero: Taiyo isn't asexual, but while he is flirtatious he is very adamant on being exclusively monogamous. Towards the end, when all three of the Miyara sisters admit to having feelings for him he suggests they share him, but is punched out by Mio and Shibuki.
- Chivalrous Pervert: Taiyo is a shameless flirt and definitely likes pretty women, but he's dedicated to being faithful to Shibuki and wouldn't dream of taking advantage of a girl or forcing them to do anything against their will. He's also a virgin and in no hurry to have sex.
- Conservation of Competence: A few guerrillas manage to give the vastly superior American special forces a good thrashing.
- Corrupt the Cutie: Happens to Mio by way of character development.
- Culture Clash: Ancient religious traditions and superstitions meet with modern secularism and values.
- Curb-Stomp Battle: The guerrillas decimate the Americans. Also, Taiyo always gets his ass kicked whenever he's in a fight, especially with Martin.
- Dawn of an Era: Subverted. The ending ends the cycle of the strange lights and symbols, but the white whale is still very much alive. At least until the final chapter, where she sacrifices herself to save Taiyo from the US Army's lost nuke. Her calf survives to carry on her legacy.
- Death Glare: Shibuki is very veeeerrryy good at giving these Taiyo, even after undergoing her ice queen defrosting.
- Defrosting Ice Queen: The Miyara sisters and Mar towards Taiyo.
- Delicate and Sickly: Mio, Lago, and Henri are afflicted by a debilitating curse that makes them catalysts for the ruins in their respective regions, which causes them to fall ill repeatedly.
- Did They or Didn't They?: It's all-but outright stated that Taiyo had sex with Mar, but he can't remember the details due to having been drugged.
- Didn't Think This Through: The consequences for doing this with SCUBA Diving are lethal.
- Disappeared Dad: Kicks off the whole plot. Also the reason why Taiyo is able to spend so much time on Zamimi.
- Disney Death: Aida and Professor Muta's ship is destroyed and they are lost at sea, but return later.
- Double Standard Rape: Female on Male: It's strongly implied Mar had sex with Taiyo while he was drugged, against his will, and the manga's ending implies she's pregnant.
- Driven to Suicide: The fate of Taiyo's mother, who sacrificed herself to keep her son from dying.
- Eldritch Abomination: The white whale is implied to be an Animalistic Abomination for much of the story, being immortal, supernatural, capable of evading the United States Navy, and seemingly appear anywhere without warning and vanish without a trace. The last chapter, where she beaches herself to save Taiyo and dies giving birth to a baby white whale, implies this is not the case, though there is definitely something supernatural about her.
- Eldritch Location: Deep water, especially caves and the mysterious ruins, are portrayed as this.
- End of an Age: Both globally and for Zamimi.
- Empathic Environment: Kinjou sacrifices Lago during a torrential downpour.
- Experimental Archeology: By the US military and Machiko.
- Fanservice: Quite a lot actually, for both sexes.
- Fanservice Pack: Mio physically matures quite a bit over the course of the story, due to her entering and progressing through puberty. Taiyo also does this.
- Family-Unfriendly Violence: During the Peru arc a soldier is shot by a rebel sniper, and then here is Lago's death
- Fille Fatale: Mio tries to seduce Taiyo on multiple occasions despite being several years younger than him.
- Forgot I Couldn't Swim: Taiyo can't swim at the start of the manga, and learns how to scuba dive over the course of the series.
- French Jerk: Henri, but becomes more endearing after some Character Development.
- Freudian Trio: Mio is the maiden, Shibuki is the seductress, and Nagisa is the mother.
- General Ripper: Jack Reynard seems to be one at first, but he was just following orders and turns out to be a decent guy.
- Genre Shift: Starts as a harem comedy and quickly turns into a mystery drama.
- Government Conspiracy: The US Government has an immense interest in the mystery and ruins.
- Handsome Lech: Taiyo's father. It turns out he's Henri's, Lago's, and Mio's father.
- He Who Fights Monsters: Overlaps with Well-Intentioned Extremist in many cases.
- Hit-and-Run Tactics: The guerrillas and most armed forces in general come out of nowhere and leave just as suddenly.
- Hollywood Healing: Averted when Taiyo and Shibuki get decompression sickness.
- Hooker with a Heart of Gold: Mar Shimabukuro prostitutes herself to get money to look after her little sister.
- Hostile Weather: The weather does not like to cooperate with anyone.
- I Am Your Opponent: Never goes well for Taiyo. He tries to fight twice in the manga. He gets beaten badly both times.
- I Did What I Had to Do: Machiko's rationalization for killing her son and Lago.
- I Let You Win: Martin gives Taiyo the win in their fight for the right to explore Peru's ruined temple... and Mar's hand in marriage.
- If I Can't Have You…: After Taiyo rejects Mio's final attempt to seduce him and stops her from committing suicide, she tries to strangle him.
- Interrupted Intimacy: Mio purposely does this any time Taiyo and Shibuki try to get it on.
- Interrupted Suicide: In the final chapter, Mio tries to drown herself after Taiyo refuses to have sex with her due to her being his half-sister. When he saves her and still rebukes her advances, she tries to strangle him, but comes to her senses and stops.
- In the Blood: Taiyo, Mio, Lago, & Henri are all catalyst and have the same father.
- I Will Wait for You: Shibuki to Taiyo before he and Mio go to Peru.
- Jerk with a Heart of Gold: Mar, Martin, and Kimjou
- Just Following Orders: Jack Reynard.
- Justified Criminal: Mar is a pickpocket and prostitute in order to get money to look after her sick sister.
- Lady Drunk: Nagisa gets drunk a lot.
- Light Is Not Good: The glowing sigil that appears on Mio's chest and the chests of several other characters, the luminous ruins, and the spectral white whale associated with them are portents of ill fortune.
- Lost Technology: The technology needed to make the ruins and the symbols work.
- Love Dodecahedron: The Miyara sisters and Mar all develop romantic feelings for Taiyo.
- The Magic Goes Away: The cycle of the lights and ruins is finally ended. And this is undoubtedly a good thing.
- Magitech: A subtle example with the symbol and the ruins.
- Missing Mom: The Miyara sisters lost their mother 10 years before the story starts.
- Mood Whiplash: Big time and right out of nowhere.
- Nosebleed: Happens whenever someone has a reaction to the ruins. Played very seriously in one instance where it is indicative of life-threatening Nitrogen toxicity.
- Past Experience Nightmare: Taiyo has nightmares several times as his trauma-suppressed memories begin returning.
- No Periods, Period: During the whale watching event Mio falls ill in a way that doesn't involve her curse, and that evening Nagisa sends Taiyo out to pick up some red rice, to his initial confusion.
- Oddly Visible Eyebrows: Pops up with everyone, but is especially noticeable on the non-Japanese characters.
- Pillar of Light: In the shape of a wierd sun/eye symbol.
- Place of Power: There are many of them all over the world.
- The Plan: The reader is kept on the outside of the various plans.
- Precocious Crush: Mio develops one on Taiyo that she makes very explicit multiple times. It doesn't go away until the epilogue.
- Quivering Eyes: During the more emotional scenes.
- Rescue Arc: Played very interestingly in that the goal isn't to physically reclaim anyone, but rather to protect them from harm.
- Really Gets Around: It's revealed that Taiyo's father did and fathered 4 children with 4 different women.
- Running Gag: Shibuki pulling boxing gloves out of hammerspace and decking Taiyo when he acts perverted. Mio gets in on it in the second-to-last chapter.
- Shoot the Dog: Machiko shoots and kills Lago.
- Shown Their Work: All the stuff about SCUBA Diving. It's heavily detailed to the point that it can get in the way.
- Surprise Incest: In Chapter 43, Mio professes to have fallen in love with Taiyo and repeatedly makes romantic advances towards him, though ultimately nothing comes of it. In the second-to-last chapter, Taiyo learns that she is his half-sister, and Mio doesn't react well to the news when she finds out, making one last attempt to seduce him and then trying to drown herself when he refuses. When he saves her, she tries to murder him in a fit of jealous rage.
- Surprise Pregnancy: It's implied Mar is pregnant with Taiyo's child, as when Machiko activates the temple she feels pain and heat in the vicinity of her uterus.
- "Take That!" Kiss: When Taiyo starts to own up to inadvertently cheating on Shibuki with Mar, Mio hushes him. When Shibuki demands an explanation, Mio tells her "What happened went something like this," and then kisses Taiyo while smirking at her sister.
- Tsundere: Shibuki starts off hating Taiyo at the beginning, but slowly and begrudgingly develops feelings for him.
- Unwanted Harem: Taiyo has a number of female characters who want to get in his pants, but is dedicated to Shibuki.
- You Can't Fight Fate: Machiko's view, which leads to Shoot the Dog
