My son recently mentioned that it’s something of a pity that costume parties in murder mysteries all have basically the same plot: two people are dressed similarly enough that they are confused. This does, of course, betray a lack of breadth in his experience of murder mysteries, but in his defense he’s only eleven years old.

To be fair, there are several basic plots to costume parties:

1. Two people have similar costumes that are confused for each other, at least in unusual lighting. (This can give the murderer an alibi or make the victim appear alive later than they were.)
2. One person dresses in the costume of another in order to frame them.
3. One person can, with a few alterations, make his costume look like another’s.
4. One person had the costume of another on underneath his own, much bulkier costume.

Curiously, I can’t recall ever seeing someone switch into another costume—that no one had come to the party in—to commit the murder, then don their original costume again. It probably exists, but this does seem pretty rare.

The main difficulty with costume parties in murder mysteries is that there is very little that one can do in a costume party that one can’t do elsewhere except for some confusion of costumes. The only other thing which comes to mind is the disguise of an unusual weapon in costume props. The difficulty there is that there are very few murders where the main difficulty is sneaking the weapon into and out of the scene of the crime, and hence being able to smuggle it in a costume is a solution to this problem, enabling the crime. I suppose one could work something up with a poisoned blow-gun, or something like that, but even there it’s not obvious why the murderer would want to keep the thing. There are too many small weapons concealed easily enough in ordinary clothing to make this really appealing.

Past that, a costume party is such a big thing that it would feel very strange for a writer to set a murder mystery in a costume party and for the murder to have nothing whatever to do with the costume party. I’m sure that’s been done because every setup in murder mysteries will also be used as a bluff, and also a double-bluff. Still, to be fair to my son, if everyone is dressed up in different costumes, it is a fair bet that someone will look like someone else, and the solution will involve figuring out who was really underneath the crucial costumes.

The real trick, as always, is making it fun.