How to play Detective Sudoku

The goal

Every case begins with an empty plan divided into rooms. Some squares contain furniture; others can hold a suspect. The clue cards describe where each suspect stood, where they did not stand or how they were positioned relative to someone or something else.

The clues are evidence, not flavour text. Every clue is true at the same time. A position that satisfies Ada’s statement but contradicts Bram’s is not a partial answer. It is wrong. The finished board must satisfy the complete file and obey the grid rule below.

You do not have to accuse anyone separately. The solved placement reveals the culprit automatically. The victim’s statement says they were alone with the murderer. That makes the final room a two-person room: the victim and exactly one suspect.

One suspect per row and column

As in sudoku, the grid has a strict uniqueness structure. Each suspect appears once. Each row contains exactly one suspect, and each column contains exactly one suspect. If Ada is placed in the third square of a row, every other square in that row can be crossed out for all remaining suspects. The same is true of her column.

This rule supplies much of the puzzle’s momentum. A clue may narrow a suspect to two squares. A placement elsewhere can close one row, leaving only the other. Conversely, if only one suspect can occupy a particular row, that row forces the placement even when the suspect still has candidates elsewhere.

Furniture changes which squares are available, but does not weaken the row-and-column rule. A table blocks a square. A chair or bed can sometimes be occupied when a clue explicitly says the suspect sat or lay there. Room boundaries describe the scene; the grid lines govern the sudoku structure.

How to read the clues

Beside someone or something

“Beside” means one square north, south, east or west. Diagonal squares do not count. Both squares must also belong to the same room. If a wall runs between two squares, the people are not beside one another even when the squares touch on the grid.

Direction clues

A suspect who was north of another suspect can be any number of rows above them, unless the clue says “exactly.” They do not need to share a column. East, west and south work the same way. Direction clues about furniture use the fixed object on the board as the reference point.

Exact-offset clues

An exact clue adds distance: “exactly two rows south of Ada” means the suspect must be two row lines below Ada. The column may still vary. Exact offsets often become decisive after another clue fixes one of the two people.

Rooms, corners and being alone

An in-room clue confines a suspect to that named room. “Alone in the study” means no other suspect may occupy any square in that room. A corner clue means a corner of the room shape, not simply one of the four outer corners of the entire grid. Irregular rooms can have several geometric corners.

Furniture clues

Furniture can be a landmark or an occupiable square. “Beside the plant” uses the plant’s fixed location. “Sitting on the chair” requires the chair square itself. Carpets are overlays rather than blocking objects, so a suspect may be “on the carpet” while still occupying an ordinary room square.

The murder rule

The victim is placed by the same rules as everyone else. Their special clue is always the key: the victim was alone with the murderer. In the final solution, the victim’s room contains exactly two people. The other person is the culprit.

This constraint is active throughout the solve. If three suspects would be forced into the victim’s room, at least one earlier assumption is impossible. If the room already contains the victim and one suspect, every remaining square in that room can be eliminated for everyone else.

Solve the room, and the accusation writes itself.

Hints that show their work

The hint system does not reveal an unexplained answer. It compares your current board with the solver’s deduction trace and finds the next logical step you have not recorded. A hint might explain that Katherine cannot be in storage because she was beside a plant, and storage has no available square beside one.

That explanation includes the clue responsible for the step. You can apply the deduction, examine the rest of the board and continue under your own steam. The goal is to restore the line of reasoning, not bypass it.

A small deduction chain

Suppose a three-by-three case has Ada, Bram and Clara. Ada says she was in the study. Bram says he was directly south of Ada. Clara says she was beside the table. The study occupies the upper-left two squares, and the table is fixed in the bottom-right room.

1. Ada has two possible study squares.
2. Only one of those squares has a legal square directly south of it, so Bram’s clue fixes Ada in the upper-left position.
3. Bram must occupy the square immediately below Ada. Their row and column can now be crossed out for Clara.
4. Clara’s remaining candidate beside the table is unique, so she takes it.
5. Each row and column now contains one suspect. All three clues are true.

On a full case, chains overlap: a room clue narrows a candidate, a direction clue removes another, and the row rule turns the remainder into a placement. Hard cases ask you to combine more layers, but the contract never changes.

Frequently asked questions