[AI Detective Manga] Chapter 29: Three Seconds After the Lightning, Glass Shatters in the Annex | How the Telemeter Exposes the "Man Who Was Far Away"

監修者 Commit Ginza
Kenji
Appraiser, this time the lightning provides an alibi.

Lightning struck the annex building 1km away. Immediately after the glass broke, a one-of-a-kind prototype watch disappeared from the main building.

Katagiri, the security manager, testified: "I was in the annex at the time. The light and sound were simultaneous."
Appraiser
Then let's ask the lightning where he was.

Light arrives almost instantly. Sound arrives delayed by the distance.
The order of arrival is more accurate than human testimony.
Kenji
Is the lightning going to clock in?

If natural phenomena start participating in timekeeping, my tardiness will be exposed on a global scale.
CASE FILE 29 The Annex 1km Away and the Missing Prototype Watch
  • Incident occurred: 20:18:12, lightning strike near the annex of the exhibition facility
  • In the main building, a loud bang was heard approximately 3 seconds after seeing the lightning flash
  • Window glass on the west side of the annex was damaged. Immediately after, an anomaly was displayed on the security terminal
  • A one-of-a-kind prototype watch with a celestial map dial disappeared from the inspection room in the main building
  • Straight-line distance between the main building and the annex: approx. 1,020m
  • No power outage. No record of the front entrance or loading dock opening or closing
  • Personnel present in the facility: 3 people

① Three Suspects. Only the "Man Who Was Far Away" Had a Perfect Alibi.

Suspect A: Shuichi Kuze 56 years old, watch collector Repeatedly inquired about purchasing the prototype watch. Testified to seeing the lightning from the main building lounge.
Suspect B: Haruka Miyashita 32 years old, event coordinator Possessed a spare key to the display case. At the time of the incident, she was on an online call at the main building's reception desk.
Suspect C: Toru Katagiri 44 years old, security manager Reported via radio, "I was in the annex 1km away at the time of the incident." Possessed master keys for both buildings.
Kenji
The culprit is Mr. Kuze.

He wants it. He can't buy it. Thunder strikes. The watch disappears.

This sequence, in a Tuesday Suspense drama, would already be at the cliff.
Appraiser
It's only five minutes in. It's too early for a trip to the cliff.

"Wanted it" is a motive. But this time, we'll first confirm who was in the main building.

② Katagiri's Testimony: "In the Annex, Light and Sound Were Simultaneous."

Katagiri's Testimony: "I was inspecting the west corridor of the annex. The moment the outside of the window flashed white, an explosive sound was heard right next to my ear, and the glass broke. The light and sound were almost simultaneous. I immediately radioed the main building."
Kenji
It's not surprising if the light and sound arrive simultaneously when you're right next to the lightning strike.

Mr. Katagiri's alibi is reinforced concrete.
Appraiser
Even if the exterior walls are impressive, there's a three-second void in the foundation.
Kenji
An alibi that's a defective house?

I'd like to cancel at the viewing stage.

③ A Telemeter Converts "Time from Light to Sound" into Distance

Appraiser
Kenji. Look at the outer scale of this chronograph.

It's not a tachymeter for reading speed. It's a telemeter for reading distance.
Kenji
Telemeter.

So it's not a scale for sending telepathy to people far away.
Appraiser
Start the chronograph the moment you see the lightning flash.
Stop it the moment you hear the thunder.

Read the approximate distance to the origin point from the outer scale indicated by the central second hand.
Kenji
Light is an express train, sound is a local train.

It's like sound stopped by a convenience store on the way, even though they left the same station.
Start with light → Stop with thunder

As a guideline, sound travels approximately 1km in 3 seconds.
3-second difference ≈ 1km away
Clue 1: Basic Telemeter Operation
  1. Observe a phenomenon where light and sound occur almost simultaneously.
  2. Start the chronograph the moment you see the light.
  3. Stop it the moment you hear the sound.
  4. Read the approximate distance from the telemeter scale indicated by the central second hand.

It is used for phenomena where the source of light and sound is the same, such as lightning or fireworks. The measured value changes depending on temperature, wind, sound reflection, scale design, etc., so it is an estimation, not a precise measurement.

④ The "Three Seconds" Recorded in the Main Building Matched the Distance to the Annex.

Two Records Remaining in the Main Building
Staff video Lightning flash: 20:18:12.40 / Explosive sound: 20:18:15.39
Appraiser's chronograph Elapsed time: approx. 3 seconds / Telemeter reading: approx. 1km
Facility blueprint Main building to west side of annex: approx. 1,020m
Lightning strike record Detected near the lightning protection equipment next to the annex
Appraiser
In the main building, it took about three seconds from seeing the lightning flash until the sound arrived.
The telemeter indicated approximately 1km.

This nearly matches the distance between the main building and the lightning strike location, approximately 1,020m.
Kenji
So, three seconds in the main building.
If you're in the annex, right next to the lightning strike, you wouldn't wait three seconds.
Appraiser
Exactly. For someone in the annex, the interval between the light and the loud sound would be very short.

Therefore, Katagiri's statement, "it was simultaneous," is natural for someone who was in the annex.
The problem is what his watch recorded.

⑤ Katagiri's Watch Also Recorded Three Seconds, Not "Simultaneous."

Appraiser
Mr. Katagiri. Please show me the chronograph on your left wrist.
Appraiser
The central second hand stopped at three seconds.
The telemeter scale shows approximately 1km.

This is the watch I reset to zero for display before the incident. There has only been one strong lightning flash since the incident.
Katagiri's Rebuttal: "I just touched the pusher in surprise. It must have been a coincidence that I pressed it again three seconds later."
Kenji
Coincidentally pressed at the light.
Coincidentally stopped at the sound.
Coincidentally, exactly 1km.

Coincidence is here to make a sales pitch as a trio.
Appraiser
A person who felt the light and sound simultaneously in the annex cannot wait three seconds for the sound before stopping.

You saw the lightning flash and then waited three seconds in the main building for the thunder to arrive.
The borrowed watch recorded that operation.
Decisive Clue for the Reader:
Katagiri testified that "in the annex, the light and sound were simultaneous." However, his chronograph recorded three seconds from the lightning flash to the thunder, and the telemeter showed approximately 1km. This matches the distance from the main building to the lightning strike location.

⑥ Verifying the "Main Building Alibis" of the Remaining Two.

Records of Kuze and Miyashita
Shuichi Kuze Continuously appears in the staff video by the main building window from just before the lightning flash until after the explosive sound. Both hands are visible.
Haruka Miyashita In the online call recording, she is seen at the main building reception desk before and after the incident. She still has the spare key around her neck.
Toru Katagiri Only his own radio report stating, "I was in the annex." No one saw him in the annex.
Kenji
Mr. Katagiri, who didn't look like he wanted it, was in the main building, more so than Mr. Kuze, who did want it.
Appraiser
Motivation doesn't always show on one's face.
Three seconds, however, showed on the dial.
Kenji
My face shows "I want to eat lunch" every day.

No appraisal needed for that.

⑦ Crime Reenactment — Katagiri Turned a "Natural Accident" into His Alibi.

Three Seconds and the Subsequent Crime Procedure
  1. Katagiri pretended to go inspect the annex but remained in the main building's service passageway.
  2. He saw the lightning flash near the annex from the main building and started the borrowed chronograph.
  3. Approximately three seconds later, he stopped the chronograph the moment the thunder reached the main building.
  4. The security terminal displayed glass damage at the annex, diverting everyone's attention to the distant location.
  5. Katagiri opened the main building's inspection room with a master key and took out the prototype watch.
  6. He radioed from the main building's stairwell, saying, "I'm in the annex."
  7. He hid the watch in the first aid kit of a covered electric cart, then moved to the annex.
Appraiser
Both the lightning strike and the glass breakage were genuine accidents.

Katagiri didn't cause the accident.
The moment the accident occurred, he pretended to be far away.
Kenji
He couldn't book a natural disaster, so he took advantage of it the moment it happened.

Only the culprit got a head start on the lightning's temporary sale.
Appraiser
However, he left three seconds on his wrist as a receipt.

⑧ The "Man Who Should Have Been Far Away"''s Belongings Emerge from the Electric Cart's First Aid Kit.

Kenji
But where's the watch?

It's not in Katagiri's clothes.
Did he perhaps throw it a kilometer away?
Appraiser
Let's check the covered electric cart Katagiri used to move to the annex.
Electric Cart's First Aid Kit
A prototype watch wrapped in dark green protective cloth was found under an unopened bandage. The protective cloth was made of the same fiber as the inspection tray in the main building. Fibers matching Katagiri's security gloves were also found on the first aid kit's clasp.
Kenji
The watch was in the first aid kit.

It wasn't the watch that was feeling unwell, it was Katagiri's alibi.

⑨ The Culprit is Toru Katagiri, Security Manager.

You Were Not in the Annex

Appraiser: "Toru Katagiri. You were the one who took the prototype watch."

Appraiser
If you had been in the annex, there would not have been a three-second gap between the lightning flash and the loud sound.

However, your watch counted three seconds and showed approximately 1km.
That distance is the distance from the lightning strike location to the main building.

You saw the lightning in the main building, and you heard the sound in the main building.
Katagiri's Confession: "An overseas broker told me that prototype machines would fetch a high price. The moment I saw the accident at the annex, I thought no one would see me in the main building. I pretended to be in the annex over the radio, then moved with the cart later. It didn't occur to me to reset the borrowed watch."
Kenji
You put your alibi a kilometer away,
but it was returned to the main building in three seconds.
Appraiser
Even if you falsify the distance, you can't change the order of light and sound.

⑩ After the Incident, Kenji Asks the Thunder for a "Delay Certificate."

Kenji
Appraiser, it flashed again.

One, two, three, four, five, six... thunder.
The telemeter shows approximately 2km.

Mr. Thunder, you're six seconds late. Please issue a delay certificate.
Appraiser
Not late. Just far away.
Kenji
Can that be used for people who say "I'll be right there" and arrive twenty minutes later?
Appraiser
Measuring that isn't done by a telemeter, but by experience.
Appraiser
(In Kenji's case, "I'll be right there" might be a concept, not a distance...)

Conclusion: The Telemeter Exposes Lies About Location Based on "How Many Seconds Later the Sound Was Heard."

Appraiser
A telemeter uses the time difference in the arrival of light and sound to read the approximate distance to the source.

In this case, in the main building, it was about three seconds from the lightning flash to the thunder. The display was about 1km, which matched the distance between the main building and the lightning strike location near the annex.

Katagiri testified, "In the annex, the light and sound were simultaneous." But his own chronograph recorded three seconds.
That was proof that he saw the lightning in the main building, not the annex.

People can change their stated location. But they can't erase the three seconds they waited for the sound after the light.
Telemeter Learned from the Case
  • Start with light, stop with sound: Uses light and sound from the same phenomenon.
  • A three-second difference is roughly 1km: Light is almost instantaneous, sound travels through the air with a delay.
  • Shows approximate distance: Varies depending on temperature, wind, reflected sound, and scale design.
  • Different use than a tachymeter: A tachymeter measures speed, a telemeter measures distance.
  • Check the scale before measuring: Specifications vary by model, whether it's km or mile display.
Function Start Stop What it reads
Telemeter When light is seen When sound is heard Approximate distance to the source
Tachymeter When a known section is started When the section is passed Average speed for the section
※Telemeter scales, units, reading ranges, and chronograph operation methods vary by model. The speed of sound is affected by temperature, wind, humidity, terrain, and reflection, so the display is an approximation. The incidents, facilities, persons, and prototype watches in this work are fictional. In situations where thunder can be heard, do not continue measuring outdoors; evacuate to a safe building.
Key Deduction Points for This Case
  • Verifying testimony about "where someone was" based on distance and arrival time.
  • A telemeter is used when light and sound originate from the same phenomenon.
  • Comparing the elapsed seconds recorded on a watch with the actual distance at the scene.
  • Investigating whether anyone took advantage of a natural accident, even if it was real.
Thumbnail Image Generation Prompt
Horizontal 16:9, full color, completely photorealistic, thumbnail for a Japanese web mystery manga. A luxurious watch exhibition facility on a stormy night with heavy rain and lightning. In the foreground, against the large window of the main building, Kenji, late 20s, with black center-parted hair, wearing a cobalt blue jacket, in a surprised expression, in the style of a Korean celebrity. In the center, the Appraiser, early 30s, with long glossy black hair, wearing a dark gray pantsuit and white gloves, with a sharp and calm expression, holding up a telemeter chronograph. The watch's central second hand is at 3 seconds, and the outer scale reads approximately 1km. In the background to the right, security manager Toru Katagiri, wearing black-rimmed glasses and a dark blue security jacket, holding a radio, attempting to hide a watch in a hurried manner. Further in the distance, an annex building being struck by lightning with broken windows. A small empty dark green velvet tray from the main building is also visible. Strong cinematic contrast, tense mystery drama, precise watch, natural hands and fingers, no extra fingers. No text, titles, speech bubbles, brand logos, watermarks, or anime expressions in the image.
NEXT CASE
Episode 30: Why was the pulse of the man who ran up seven floors 72?
〜The Pulsemeter Exposes the "Alibi of Mere Breathlessness"〜
  • The suspect testified that he "ran up the emergency stairs at full speed."
  • Immediately after arrival, despite heavy breathing, the watch's measurement was strangely calm.
  • A pulsemeter, which calculates a minute's heart rate from a fixed number of beats, detects feigned exhaustion.
Next, Kenji aims for athlete certification just by deep breathing!
*Pulse rates vary by individual. Next time, we'll deduce based on multiple pieces of evidence.

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