I reread this before seeing an operatic version, though I had read it several times before.
The plot is grim but simple. A traveller to a tropical penal colony is invited to watch their unique method of execution.
The machine is explained in graphic detail by the officer who has devoted years to its upkeep and worships it almost as much as he worships the previous commander who invented it. The officer is despairing that the new commander is not enlightened enough to give full support to the method and fears the commander wants to abolish it.
Although proud of the moment of enlightenment bestowed just before death, the officer's concern is mechanics, not morality. Was it madness that made him so obsessed, or his obsession that drove him to madness?
The traveller is horrified, but is unsure whether (or how) he should intervene.
The particularly Kafkaesque element is that the condemned man is unsure of his crime, has had no opportunity to defend himself, does not know he is due to be executed and does not speak the language of the officer, traveller or guard.
The more surprising aspect is the fact that followers of the previous commander believe that one day he will "rise again" from his grave...
See my Kafka-related bookshelf for other works by and about Kafka (http://www.goodreads.com/review/list/1199525-cecily?format=html&order=a&shelf=kafka).
See Bruce's excellent review for an interesting angle, seeing this as a metaphor for language and communication: http://www.goodreads.com/review/show/142789452