"Isle Of Hidden Treasure Murder Case" (aka "Hidden Isle Murder Case") is the fifth case arc in the Files Series of The Kindaichi Case Files.
Hajime gets coaxed by his mother (and Miyuki) into taking the qualifying exam for a treasure hunt expedition at Hihou Island, where whomever can find the island's mythical treasure gets rewarded with 200 million yen. Hajime, of course, passes the test and gets to come to the island with Miyuki as an attendant, alongside a group of treasure hunters vying for the money. As soon as they make landfall, however, the expedition's immediately derailed when the host is found dismembered in a grandfather clock inside his own home - the supposed work of a mythical monster who hunts down all who seek the treasure of Hihou Island.
Tropes include:
- Accidental Pervert: Kindaichi got the vital clue that allowed him to Pull the Thread by walking in on the culprit just as "she" was using the bathroom—he realizes later that "she" had left the toilet seat up.
- Attractive Bent-Gender: The culprit is actually a 13-year-old boy disguised as a 17-year-old girl. He's certainly pretty enough to get Kindaichi to be smitten with "her" before Kindaichi discovers the culprit's real identity.
- Bait-and-Switch Gunshot: The intendant turns out to be one of the people who were responsible for the death of the culprit's father, and he holds the survivors at gunpoint late in the case arc, with the intention of killing all of them and taking the treasure for himself. He aims his weapon at Kindaichi, who closes his eyes... and then a shot goes off, with Kindaichi opening his eyes upon realizing that he's still alive and witnessing the intendant being injured and disarmed. It's at this point that Kaya, one of the participants in this treasure hunt, reveals her status as a cop by showing her police ID and a gun being hidden in her box.note
- Be Careful What You Wish For: Zigzagged. Even after being disarmed, the intendant insists that he wouldn't get caught by the police and that the treasure would be his. He gets the first part of the wish when the contraption being set up around the treasure gets activated after he bumps into it, which kills him on the spot.
- The Cake Is a Lie: The treasure hunt is actually part of the culprit's murder plot against the people who were responsible for his father's death, and everyone else who's selected has a role the culprit has in mind in said murder plot. That being said, this trope is played with in this case, as the treasure is real — the culprit just doesn't care for it.
- Contrived Coincidence: Part of the backstory involves the culprit and the daughter of the owner of the Hihou island estate meeting by pure chance. The culprit, after murdering his uncle for his years of abuse and running away from an orphanage where he lived for the next couple of years, lived in the streets as a homeless youth. The girl, in the meanwhile, was studying aboard at the time. She saw him on a street in New York City, a large city with millions of people.
- Dead All Along: What the fate of the 17-year-old girl the culprit disguises as turns out to have been, as the result of her father's Parental Neglect due to his obsession to treasure-hunting proving too much for her to handle anymore and causing her to become Driven to Suicide.
- Dead Person Impersonation: The 17-year-old girl the culprit disguises as (and an Attractive Bent-Gender, at that) turns out to have been Dead All Along.
- Eat the Evidence: Everyone is served chicken for breakfast on the second day of the stay on the island. It turns out that the culprit serves chicken to everybody in order to get rid of some of the evidence proving that chicken blood has been used for the crime scene.
- Fiery Cover-Up: The cabin where one of the victims is found has been burned down. The reason for burning down the cabin after the victim's body has already been chopped up into many pieces turns out to be to prevent the investigative police from discovering that the blood at the scene belonged to chicken instead of the victim, as Kindaichi reveals during The Summation.
- He Knows Too Much: The reason behind the murder of the former coroner, who is not among the culprit's intended victims, is because he had seen the culprit in the process of changing into a girl's clothes.
- Hypocritical Humor: As Kindaichi and Miyuki arrive at the test site for the qualification exam for the treasure hunt, Miyuki remarks that everyone is eager to get their hands on the treasure, only for Kindaichi to retort with "so are you".
- Not the First Victim: The owner of the island estate, whose dismembered corpse is witnessed by everyone soon upon arrival, turns out to be the second person the culprit has killed. The first victim turns out to be the culprit's own uncle, whom he murdered years ago after suffering abuse at his hands, as the culprit reveals regarding his own backstory.
- Parental Neglect: The 17-year-old girl's backstory involves this, as her father was so obsessed with treasure-hunting that he basically abandoned her after making sure she didn't starvenote . He never changed his attitude even when inviting her for the treasure-hunting quest on Hihou Island, which was the last straw and caused her to become Driven to Suicide.
- Pragmatic Villainy: The culprit saves Kindaichi from a certain death when the latter is tricked (and not by the culprit, mind you) into nearly getting into range for a steam geyser eruption with the temperature of 120°Cnote , though this is only because Kindaichi is still needed for the culprit to trick into pointing to another participant as the designated "murderer".
- Rape as Backstory: This is part of the culprit's backstory, as it turns out that, after the death of his father, he was adopted by his uncle, who, in addition to forcing him to work at his ranch and mistreating him, forced him to dress like a girl and tried to rape him, until he got fed up and got his revenge by slipping him some sleeping pills into his uncle's drink before setting his house on fire, killing his uncle as a result. This was his first murder, and the police didn't suspect him simply because he was nine years old at the time.
- "Shaggy Dog" Story: The whole plan behind getting Hajime into the treasure hunt is to have a share of the treasure, and Mrs. Kindaichi also promises a part of the share to Miyuki should she succeed in the quest. As the case ends, however, Kenmochi states to Hajime and Miyuki that the treasure will be donated to the Japanese government after the owner's demise, which means no one in the Kindaichi and Nanase families gets any share of the treasure.
- Tempting Fate:
- A positive example that is Played for Laughs. Hajime's mother has misgivings about her son's puzzle-solving capacity, and Miyuki wonders during the exam whether he will really get qualified for the treasure hunt after seeing him leaving early. Hajime ends up being qualified for the quest.
- A much more fatal version happens to the intendant late in the case arc, when he tries to kill all of the survivors and take the treasure for himself, only for Kaya to injure and disarm him with a shot of her own. He claims that he wouldn't get caught by the police and that the treasure will be his, but one accidental bump into the treasure in question activates the trap being set around it and gets him killed on the spot. At least he manages to fulfill his wish of not getting caught -- alive, that is.
- Wardrobe Malfunction: Kindaichi does it to the culprit by tearing off a part of the dress, which reveals a stuffed bra falling off onto the floor and the culprit's blatantly male body underneath.
- Yank the Dog's Chain: As the culprit confesses regarding his backstory, he stayed in an orphanage after murdering his uncle for his years of abuse until he ran away from it, resorting to theft, robbery, and scam in order to survive as a homeless youth. Sometime later, the daughter of the owner of the island estate encountered him by pure chance -- bear in mind that they were both in the populous New York City at the time — and befriended him at once due to having met him years ago. Though he initially planned to target her as Revenge by Proxy when the timing was right, her disclosure of her own Parental Neglect and her genuine kindness got him to change his mind and actually start enjoying life a bit more. All of his newfound appreciation of life was taken from him when she was Driven to Suicide after her father's invitation letter arrived, which fueled his resolve to murder the people who were responsible for not only his father's death but the suffering that she and he were put through as well.
- You Are Too Late: After the 17-year-old girl received the last letter from her father, she became Driven to Suicide because the letter was only an invitation for the treasure-hunting expedition on Hihou Island, where the murder case arc would unfold later, with NO warm words of kindness throughout the letter content from him, and the girl, who was already emotionally fragile at the time, found the further evidence of Parental Neglect too much to handle at this point. She was already dead by the time the culprit discovered her body.
