Pick your case
Browse the shelf. 1920s Egypt, 1930s India, 1950s Mexico, post-war Europe — pick the one that calls you.
Designed for the serious sleuth From £45 · posted worldwide
Solve a cold case
at home.
Cosykiller is a hand packed case file which strives for authenticity and immersivity. Fiction set in the real world. Each case leads you through a snap shot in time and is packed with replica letters, photographs, telegrams and other evidence designed to satisfy the most detailed detectives.
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5.0 from hundreds of sleuths Verified reviews from solved cases
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Free UK shipping over £75 Tracked international post worldwide
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Hand-packed in Grimsby Dispatched within 3 working days
Awaiting Your Investigation.
Pique your curiosity.
Each case file contains enough detail to occupy even the most serious of detectives. Average solve time is listed per case. Some are episodic, others single box solves. All are narrative fiction with complex characters and detailed plots.
Case № SHADO-012-4 hours
Europe 1946
Shadows in the Gallery
A body discovered in a bricked up cellar unlocks the start of a cold case investigation.
£45Open the case
Case № SITEQ-019-12 hours
Mexico and America 1950s-1980s
The Secret of Site Q
A body uncovered at an abandoned campsite in the dense Mexican rainforest reveals there was more to a simple tragedy.
£285Open the case
Case № INHER-016-11 hours
Simla, India 1934
An Inheritance of Murder
An heir hunter team reaches out to you to ascertain if you are the long lost heir of an English Estate.
£285Open the case
What's in the box
Real World Setting,
Fictional Stories.
Every case is different, and we want to avoid spoilers, but across all our Cosykiller Cases you'll find four ingredients woven into each plot making our stories unique and immersive.
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Clues & Ciphers
Letter substitutions, knock codes, Vigenère, one-time pads, invisible ink. Unlock each code to reveal more.
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Puzzles
Fold-out maps, layered photographs, cryptic exchanges, the occasional burnt page. Hand-assembled.
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Mementoes
Dried botanicals, trace evidence, period spices, pendants and coins. Little objects that change how a case reads.
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Sketches & Photos
Portraits, photographs, letters, military archives, newspaper clippings…everything comes together to provide immersion.
How it works
Three steps to solve your first case.
We post the box
Hand-packed in Grimsby. The full case file — letters, photographs, maps, physical evidence — sealed, wrapped in brown paper, posted worldwide.
You work the case
Pour a drink. Clear a table. Read. Cross-reference. Decide who did it. Break the final seal when you're sure (or nearly sure).
Buying for someone else?Posted direct
A case, hand-packed, addressed to them.
Enter their address at checkout and we'll post the box direct — the receipt only goes to your email. Add a handwritten gift note at order and we'll tuck it inside the opening envelope. Perfect for birthdays, Christmas, bookish hen-dos, retiring detectives.
- Ship to a different address
- Handwritten gift note
- Receipt hidden from the recipient
What our customers say
“I received my first box in mid December and have only just sat down to start reading — but I can already get a sense of fantastic attention to detail, from the turmeric pages to the chai tea to the book cover and the faded bits. Can't wait to begin to make sense of it all.”
“Cosy Killer is by far the best murder-mystery box. If you're looking for an engaging experience filled with authentic characters, interwoven plot lines and puzzles organically entwined within the narrative — this is it. The team's skills encourage an inevitable emotional investment. There's a point where you decode a tricky artefact and with every letter my heart ached a little as I could predict what the next letter would be.”
“I recently completed "An Inheritance of Murder" and I am officially in awe of the folks at Cosy Killer. The storyline was intriguing and the materials in each box were elaborate and contributed to the aura of mystery. The puzzles, clues and ciphers were varied and complex — and if you get stuck there's a community of nice folks to brainstorm with.”
Common questions
Before you ask
What is Cosykiller?
A hand-packed murder-mystery case file delivered to your door. Each box contains clues, keepsakes, ciphers and mementos — like an Agatha Christie novel where you play Miss Marple. No conclusions are drawn; the puzzling and detection is up to you.
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. Shadows in the Gallery is our advanced case, layering playfair, knock code and Caesar ciphers across a post-war art theft — do it 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. Every case is rated 14+ for themes of murder, and Shadows in the Gallery touches on wartime violence and post-war trauma, so we'd steer younger players towards An Inheritance of Murder first.
Can I buy a case as a gift?
Yes. At checkout you can enter a different shipping address; the receipt only goes to the billing email. If you'd like us to include a handwritten gift note, drop a line to <a href="mailto:hello@cosykiller.com">hello@cosykiller.com</a> with your order number and the message.
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 like photographs, ciphers, maps, letters, military archives, telegrams, postcards, and other personal effects collected during the investigation. Some cases include dried botanicals, spices, theatre programmes and small physical objects. Each case is a complete story, with the final solution provided at the end. Nothing to download or sign into.