Alone With the Victim

Murdoku has two rules. The first — a row and a column for every person — drives almost everything on this page. The second is the one that makes it a murder: the victim was alone with the killer. The room the victim is found in holds the victim and exactly one suspect, nobody else. That suspect is the murderer.

It is easy to treat the second rule as the punchline and forget it while solving. That is a mistake. It is a constraint like any other, and in about a quarter of the casebook it is needed to finish the case at all.

Three Ways It Cuts

First: a room that already holds two suspects for certain cannot be the victim’s room. Two is one too many. Every tile of that room is crossed out for the victim.

Second: once the victim’s room is known, anyone certain to be in it is the murderer — and since there can only be one, every other suspect leaves that room. A whole room disappears from everyone else’s list in one step.

Third: if only one suspect can possibly be in the victim’s room, that suspect must be there. The victim cannot be alone, so the room needs its murderer, and there is only one candidate.

The Victim Is a Person Too

The victim has no clue of their own, but they still take a row and a column. Often the victim is placed by elimination: once the suspects have taken their lines, one row and one column are left over, and the victim stands where they cross. In the casebook that is the very last placement in nearly half of all cases. The verdict follows directly — whoever shares the victim’s room is the murderer.

Only the victim’s gender is ever shown, and no clue depends on anything else about them. If you find yourself reasoning about the victim’s hair or glasses, something has gone wrong; those are hidden on purpose so that no case can hang on a detail you were never given.

A Worked Example

From the case A Little Murder Among Friends — 4×4, Medium, House & Home.

C1C2C3C4
R1Ch+CT
R2C
R3Ch+CC
R4P
The floor plan of “A Little Murder Among Friends”. Letters mark objects: C Carpet, Ch Chair, P PC, T Toilet.

The clues in play:

This step comes before anyone in the case is placed, so reading it does not give the solution away.

At step 5 of the solve, the reasoning reads:

The Bathroom is exactly full (victim alone with the murderer) – one required cell there can only be Anna.

Cases That Need It

142 of the 142 cases in the casebook use this technique at least once. A selection, smallest board first:

Other Techniques

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