How to solve a murder mystery sudoku: the techniques that always work
You have the floor plan, you have the cards, and you have spent ten minutes staring at the grid without placing anyone. This is not about luck or intuition: in a well-built case there is always a safe next step waiting, and it is almost always one of these five. Here they are, in the order you use them.
The two rules everything else comes from
One person per row and per column, and every card tells the truth. Everything else follows from that.
Before the techniques, the reason they work. A murder mystery sudoku rests on two rules, and only two.
The first is the sudoku one: one person per row and one per column, never repeating. Everyone counts, the victim included. And some squares cannot hold anyone, the ones taken up by furniture: they are shaded and out of play.
The second is that the cards tell the truth. Each suspect signs one sentence about where they were — in a room, on an object, in a row, next to another person — and that sentence is true. When a case includes a liar, it warns you before you start.
From those two comes the part that matters: the case has exactly one solution and you can reach it without guessing. If you are trying things out to see what fits, there is a safe step you have not spotted yet.
1. Start with the most specific clue, not the first one
Rank the cards by how much they narrow things down, and place the one that leaves a single square.
The beginner's mistake is to read the cards in order and attack the first one. Read them all and rank them by how much they narrow things down.
At the top go the ones that leave a single possible square: "I was on the rug" when the plan has one rug, "I was in the armchair", "I was in the corner". Those place themselves, with no deduction at all.
Then the ones that leave two or three: "I was in the kitchen", "I was in the first column". And last, the ones that talk about somebody else: "I was next to Bruno" is worth nothing until Bruno has a place.
Place the most specific one and read the list again. The ranking changes after every placement, and that is exactly what moves the case forward.
2. The cross: every placed person empties a row and a column
Every person placed crosses out their whole row and their whole column.
This is the technique that does the most work and the one people forget. The moment somebody has their square, cross out the whole row and the whole column: nobody else can be there.
Cross them out for real, on the board, not in your head. Every cross you draw shrinks everyone else's options, and cases fall to an accumulation of crosses, not to flashes of inspiration.
The flip side is just as useful: if a row or a column has only one free square left, somebody has to be in it. You do not know who yet, and it does not matter: mark it as taken and carry on. The name turns up later, once the cards have closed in around it.
3. Rule out by room, and read the general clues twice
“Next to” means adjacent and in the same room: walls cut.
Before wrestling with individual squares, ask who can be in each room. You get further in blocks than square by square.
This is where the general clues earn their keep — the ones at the foot of the case that belong to nobody: "there were only women on the pier", "nobody was next to a rock". They place no one, but they empty whole rooms for half the suspect list, and they are usually what unblocks a case that looks stuck.
And one detail that costs points until it sinks in: "next to" means the adjacent square — above, below, left or right, never diagonally — and inside the same room. Two touching squares separated by a wall are not next to each other. That detail only ever rules out placements that looked right, which is precisely what you need.
4. The locked pair: two squares in the same row
Two possible squares in the same row: that row is already theirs.
The intermediate technique, and the one that separates people who finish the easy cases from people who finish the hard ones.
If a suspect has only two possible squares left and both are in the same row, you still do not know which one they are in — but you already know the row is theirs. Nobody else can use it: cross out the rest of that row for everyone else. Two squares in the same column work the same way.
It is the classic sudoku argument, and it works without placing anybody. When a case stalls and there is no forced square in sight, what is usually missing is a locked pair nobody has looked for.
5. The victim closes the case
The victim takes the last free square; whoever shares their room is the murderer.
The victim plays by the same rules as everyone else: one square, one row, one column. In practice that makes them the last piece: once everybody is placed, the victim takes the square that is left.
It also works backwards. If at some point only one free square remains and you have not placed anyone there, you already know where the victim was — and with it the room where it all happened. Whoever shares that room with them is the murderer.
So it pays to look at the ending before you get to it: sometimes a case unravels from the back, from the only room that can host the crime.
And if you are still stuck
Crosses, general clues, rows with a single free square, locked pairs. In that order.
Run through this list: have you crossed out the row and column of everyone placed? Have you reread the general clues after your latest placements? Is there a row or column with a single free square? Is there a locked pair?
If you are still stuck, the website has progressive hints: they give you the next step of the reasoning, with the why, not the answer. You learn more from one hint you read than from half an hour of trial and error, and some cases have their solution explained on video.
Where to practice these techniques
They sink in best in pencil, and that is what the book is for. Sudoku Detective is a paperback, in full color, with illustrated settings and the solution to every case explained step by step: it does not hand you the finished grid, it walks you through the reasoning that gets there, which is the part that stays with you. The cases in the book are its own — they are not on the website.
To practice today there are free cases in your browser, no sign-up and nothing to install, with new cases every week; printable cases whose solutions come explained in a separate PDF; and the full rules with one case solved end to end, in case you skipped one.
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Frequently asked questions
Do I need to know how to play sudoku?
No. Only one rule comes from sudoku: one person per row and one per column. Everything else is deducing from what the cards say, and you pick that up in your first case.
Can you solve it by trial and error?
You do not need to, and it is the slow way. A well-built case has exactly one solution and there is always a safe step available: if you are guessing, there is a deduction you have not spotted.
What exactly does “next to” mean?
The adjacent square — above, below, left or right, never diagonally — and inside the same room. Two touching squares separated by a wall are not next to each other.
Where do I start on my first case?
With the card that leaves a single possible square, usually the one naming a unique object. Place it, cross out its row and column, then reread the other cards: one of them has almost certainly become specific.
Does the victim count for the row and column rule?
Yes. The victim takes a square like anyone else and respects their row and column, and is usually the last one placed. The murderer is whoever shares a room with them.
Where can I practice with explained solutions?
In the Sudoku Detective book, which explains every case step by step. The website also has progressive hints that give the next step of the reasoning rather than the answer.