Pick your case
Browse the shelf. Victorian Whitechapel, 1920s Egypt, 1950s Mexico, 1930s India, 1800s South China Seas — pick the one that calls you.
Case № 001–005Filed · Shipped worldwide
A murder in a box.
Posted to your door.
Cosykiller is a hand-packed case file from a single unsolved murder. Letters, photographs, telegrams, evidence. You work the papers. You name the killer. Everything you need is in the box.
Free UK shipping over £75
Hand-packed in Grimsby
14-day returns on unopened cases
Currently on the shelf
Five cases, ready to post
Six to eleven hours of mystery per box. Period paper, sealed answers, the complete case file of a single unsolved murder. Hand-packed in Grimsby and posted worldwide the moment it leaves the workshop.
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.
£109
Open the case
Case № HIDDEN-01Devilish
Interwar Britain, 1930s
The Hidden Listener
An interwar radio operator. A coded knitting pattern. And somebody in the room who shouldn't have been listening.
£99
Open the case
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.
£135 £119
Open the case
What's in the box
Four decades of murder in paper form
Every case is different and we won't spoil it — but across any Cosykiller file you'll find four ingredients, assembled to work together.
-
Clues & Ciphers
Letter substitutions, knock codes, Vigenère, one-time pads, invisible ink. Every technique taught inside the box.
-
Puzzles
Fold-out maps, layered photographs, cryptic exchanges, the occasional burnt page. Hand-assembled, never templated.
-
Mementoes
Dried botanicals, trace evidence, period spices, buttons and coins. Little objects that change how a case reads.
-
Sketches & Photos
Portraits of suspects, evidence photographs, architectural plans and contact sheets — printed on the right paper.
How it works
Three steps from a post box to a verdict
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
(And no — we didn't pay them)
“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. 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.
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, 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.