In the English village of South Mardian the winter solstice is marked by a complex ritual dance performed solely by men. This year, not long after WW II, the major participants in the dance are the local smithy and his four sons - who have a long ancestry in the area - a village doctor, a parson's son/lawyer, and a former army officer.
The story acted out in the dance is described as resembling 'King Lear' because it involves children (in this case sons) at odds with their old father. The superstitious ritual includes donning elaborate costumes and dancing, reciting, jingling, chasing girls, and brandishing swords. Toward the latter part of the ritual one of the sons beheads the father, who sinks out of sight behind a boulder. Then, at the dramtic climax, the father is supposed to come back to life and jump up from behind the rock. This time, however, the father doesn't pop up. It turns out he's actually been beheaded.
The entire village is on hand to watch the ritual dance, including an overbearing dowager and her eccentric niece, a sexy barmaid, a pretty young acting student, etc. Also present is a German visitor - a woman who studies and writes about English folklore/folk dances. The lady is regarded with suspicion by some villagers, both because she's German and a woman.
When Detective Roderick Alleyn shows up to investigate the murder he asks each dance participant and a number of observers to describe - in excruciating detail - all aspects of the dance. Every witness claims that no one went near the boulder shielding the old man between the time he was 'beheaded' in the dance and the time he was supposed to rise up again.
The questioning of witnesses takes up a large part of the book and is exceedingly repetitive and tedious. Moreover, since I didn't actually see the dance and am not familiar with British folk dancing, the descriptions were difficult to follow. And finally, when the murderer and modus operandi were exposed I couldn't picture it and it didn't make sense.
In the time covered by the story various other things are going on in South Mardian. There's a 'Romeo and Juliet' type romance (the relatives don't approve); the German woman acts peculiar and fears the police; there's pressure on a couple to marry (against their wishes) because they were seen canoodling in the forest (a product of those conservative times); the smithy and his sons have real life arguments; etc. The characters, however, are not well fleshed out and not terribly interesting.
This is not one of Ngaio Marsh's best books. It seems more like a book about British folk dancing than a mystery. I wouldn't recommend it.