"Antlion Trench Murder Case" (aka "Hidden Isle Murder Case") is the fifth case arc in the Returns Series of The Kindaichi Case Files.
Hajime and Miyuki get an offer from Itsuki to participate in a three-day trial conducted by Saito University's Psychology Professor to find a correlation between color and inner trauma/desire in a research facility called "Antlion Trench" due to how said facility is surrounded by quicksands. However, not even a day into the research, one by one the participants are found dead in the maze-like facility with the culprit seemingly vanishes into thin air. With nowhere to go since the guide to the facility is pulled before the murder begins, Hajime has to solve the murder before more victims fall.
Tropes include:
- Anonymous Benefactor: The research is requested to the professor by a shady institution with 1 million yen as payment and the research methodology/supplies already provided with a specific request to include Itsuki, Hajime, and Miyuki into the research. Three guesses on who this institution belongs to.
- Bad Samaritan: After the culprit was informed of her miscarriage while in hospital, one stranger came forward offering help. Unlike the swimmer who saved her from suicide, however, this one had a self-centered ulterior motive in mind.
- Continuity Nod: Itsuki's hydrophobia and its relation to Lake Hiren Murder Case is mentioned and used as the basis of his trauma color.
- Deadly Prank: The intended murder targets had instigated a prank on a man toward whom they felt resentful, with the prank in question being to put super-absorbent polymer in the guy's life jacket when he was due to snorkel the next day. It went horribly wrong and caused the guy to drown.Professor: Super-absorbent polymer increases 50 times in weight when it absorbs water. Even the best swimmers will sink like that!
Surviving Target: I honestly didn't think it would end so badly! Even more, our boat engine suddenly didn't work... - Downer Beginning: This case arc starts with the illustration of a group of men finding a cadaver having been washed ashore, one year prior to the present-day plot-line.This event would be the start of a tragedy that would unfold one year later.
- Driven by Envy: Two of the three intended murder targets acted out their envy on a guy with greater success with a prank that turned deadly quickly.
- Hope Spot: Played for Laughs. Early in the case arc, Miyuki notices Kindaichi writing furiously during a math class, calculations and all, and goes to talk to him after class... only to discover to her disappointment that his calculations have been about how much the mobile video game he wishes to buy would cost and whether his allowance would be able to cover the potential cost.note
- Interrupted Suicide: This is part of the backstory of the culprit, who, in the moment of Despair Event Horizon, decided to jump off a short cliff and drown, only to end up being saved by a snorkeling swimmer.
- Not Good with Rejection: The lone girl in the trio who plotted a prank that ended up unintentionally deadly on the guy they were targeting went along with the other two because she didn't appreciate the guy turning her down and was hellbent on having him humiliated for it.
- Parental Abandonment: The start of the culprit's tragic backstory all began when a client betrayed the culprit's father, who frantically tried to keep the family business going, culminating in the culprit's father being in prison after stabbing the client's director in a fit of rage, and the culprit's mother, whose heart condition was already frail, suffered a fatal heart attack after being told of her husband's arrest, back when the culprit was just a high school student.
- Playing Possum: The culprit uses a variation of this in the second murder, and intends to do the same in the third one. The culprit poses as the fist victim placed in the hallway leading to the second victim's crime scene to hide themselves. Since the corpse is covered by a blanket and no one dares to take a peek at said corpse when there's a potential victim, it's probably the best hiding spot in that scenario.
- Remote Vitals Monitoring: The Psychology Professor monitors his subjects using a wristband that sends records of the user's vital signs every 5 minutes to see how they react to trauma. The bracelet also lets the head researcher knows when someone dies. This bracelet is also used by the culprit as an alibi to buy time.
- The Shut-In: One of the research subject is a loner. While the isolated nature of this research makes the others uncomfortable, she enjoys the solitary it gives to her.
- Trauma Conga Line: The culprit's suffering began with the father being betrayed by a client and then imprisoned for stabbing said client's director in a moment of rage and the mother dying of a fatal heart failure after receiving the news of her husband being arrested for murder, leaving the culprit — a high school student, at that — with a mountain of debt until the burden proved too much to handle, at which point the culprit decided to end it all by jumping off a cliff to drown, only for it to end in an Interrupted Suicide because a snorkeling swimmer happened to be around at the time. The swimmer was soon in love with the culprit, who was far more hesitant due to the family history, but the former was undeterred even after discovering said family history, intending to marry the culprit. Unfortunately, the culprit's hope was dashed after the swimmer died from a Deadly Prank — not long after the culprit had become pregnant, at that. The culprit passed out while walking after attending her fiancee's funeral and was hospitalized, at which point she was informed that she suffered a miscarriage. One person offered to provide help, but it turned out to be a Bad Samaritan, who is also the Anonymous Benefactor and the criminal mastermind in this case arc.
- Yank the Dog's Chain: As the backstory went, the culprit, after being saved in an Interrupted Suicide, fell in love with the swimmer who never wavered on his love towards the culprit even after knowing the culprit's dire financial status, but the hope for a better life was dashed after swimmer died from a Deadly Prank.
