Case № HIDDEN-01Interwar Britain, 1930s
The Hidden Listener
An interwar radio operator. A coded knitting pattern. And somebody in the room who shouldn't have been listening.
The case
The Hidden Listener
The 1930s. A post-office telegram falls off a sorting room desk and ends up in a wastepaper basket — except someone fishes it out, and someone else notices. Morse in the knitting patterns. Knock codes in the walls. A dictionary that isn't quite the one it claims to be. A case for supersleuths who know their ciphers cold.
Nobody meant for the telegram to be read. It came off the sorter’s desk half-folded, landed in a wastepaper basket, and by the next morning somebody — we don’t know who yet — had retrieved it.
Your case file is what they left behind. The retrieved telegram, the dictionary they shouldn’t have had, the knitting pattern with the odd stitch count, the four letters that don’t quite add up. You know how this works: follow the paper, not the people, and watch for the stitches.
Before you order
The Hidden Listener is one of our advanced cases. Work An Inheritance of Murder or The Curse of Humanrah first if this is your first Cosykiller.
The envelope contents
What's in the box
We do our best to theme the contents to the story. This means the box will include interesting objects like dictionaries, knitting patterns and other memorabilia. Most of the narrative part of the story is contained in different paper documents which include telegrams, personal letters, timetables, photograph fragments and more.
- № 01
Post office telegrams
Coded cables routed through an ordinary sorting room.
- № 02
A knitting pattern
Morse-coded rows concealed in the stitch count — you'll need a pencil.
- № 03
A foreign-language dictionary
Not quite what it says on the cover.
- № 04
Correspondence
Letters between four agents and two innocents, one of whom is lying.
- № 05
Physical evidence
A wax seal, a torn timetable, a scrap of photograph.
- № 06
The listener's notes
Whoever this is, they have been writing it all down.
Audience & difficulty
Challenging but solveable.
1930s
An advanced case built for supersleuths who know their ciphers cold. Expect Morse in non-obvious places, knock codes in the walls, Vigenère exercises and a dictionary that isn't quite the one it claims to be. Work at least one Cosykiller case first if this is your first.
Ciphers you'll meet
- Morse
- Knock code
- Vigenère
- Substitution dictionaries
- Hidden-text patterns
From the case
A closer look
Letters of praise
What detectives are saying
“Tense and inventive. The transcripts and surveillance ephemera make this one feel different from the rest — you really are piecing together who said what, when.”
“Worked this one alone over four nights and still got the same thrill on each box open. The audio-era detail is beautifully researched.”
“Didn't want it to end. There's a late-game document that genuinely made me stop and re-read an earlier one — rare for this kind of thing to earn that kind of reaction.”
Ready?Case № HIDDEN-01
Post me a killing.
Other cases
More on the shelf
Case № MARL-01Devilish
Whitechapel, London, autumn 1888
Murder at Marlborough House
Victorian Whitechapel, 1888. Not the one you're thinking of — but the same streets, the same fog, the same kind of quiet cruelty.
Case № CHING-01Devilish
South China Seas, 1810
The Legend of Ching Shih
South China Seas, 1810. The most successful pirate in history vanished with a fortune. Somebody wrote it all down, and the paper survived.
Case № SITEQ-01Classic
Mexico, 1953 · reopened 1973
The Secret of Site Q
An archaeological expedition. A member missing. A black market awakening twenty years too late.
Before you buy
Common questions
I'm new to Cosykiller — where should I start?
Start with An Inheritance of Murder. It's our gentlest case, built for puzzle-minded beginners — Braille, letter-to-symbol substitutions and one-time cipher pads. The Curse of Humanrah is the next step up with knock codes, Vigenère and occasional folding. The Secret of Site Q sits in the middle — challenging but solvable. The Legend of Ching Shih and Murder at Marlborough House are our advanced cases — do them once you've worked one of the others.
How old do you need to be to play?
Cosykiller is written mainly for adults, but families can participate together. A note of caution: Murder at Marlborough House is set in 1888 Whitechapel and contains graphic descriptions and period drug references — 18+ for that one.
What's inside a Cosykiller box?
It varies by case, but every box contains a cover letter from Fairhall & Brett Inheritance Recovery — the fictional investigators who brief you on the case — followed by the documents, photographs, ciphers, maps and personal effects collected during the investigation. Some cases include dried botanicals, spices, telegrams, theatre programmes and small physical objects. Each case is a complete story in a single box, with the final solution sealed inside. Nothing to download or sign into.
How long does it take to solve a case?
Between six and eleven hours, depending on the case, the number of players, and how much you like to re-read. Most people spread a case over two or three evenings. You can pause whenever you like — put the lid back on, come back next weekend. Nothing expires.
Can I play solo, or do I need a group?
Both work well. Every case is designed to be solvable on your own — no mechanic depends on a second reader. That said, two to four players is where a case shines, because half the fun is arguing civilly about motive over a drink.
I'm stuck on a code — what do I do?
Each case comes with three sealed hint envelopes — open them in order when you get stuck. There's also an online community where other detectives swap ideas (link included with your order). If you'd rather not break a seal, email <a href="mailto:supersleuth@cosykiller.com">supersleuth@cosykiller.com</a> with your case number and the document you're stuck on and we'll help without spoiling.