It's short, barely a novella. The book begins with the narrator introducing himself, in the chapter entitled Introduces Himself. The character is the cliché of the useless British upper class remittance man. He is a bachelor and expert croquet player who lives with his wealthy aunt and is realistically aware of his own shortcomings. It is actually quite a funny portrayal. The story begins with our narrator sitting on the terraces at Les Noupets, "nibbling a brioche and consuming a harmless vermouth and seltzer." He notices a man at the table next to him furiously flipping through book after book. They being to converse and it is the story of the other man that is the main narrative of the book. He is a middle-class doctor, who due to some stress decided to open a practice out in the country. He moves to Cainsmarsh, an isolated rural area near a large marsh. He discovers the locals to be suffering under some general anxiety and fear, feelings which soon start to invade his own consciousness. Violent crimes occur in Cainsmarsh at a much higher rate than would be expected. People are excessively cruel to their animals. The doctor becomes more and more unhinged, but decides to try to investigate, as is the duty of a rational man.
I'm not going to give away spoilers. There are themes here we find in some of Wells earlier work about man and his relationship to the beast of the wild. The Croquet Player, though, is more explicitly horror in its stylings, actually quite creepy. I always love Wells' english and reading this book reminds me that I need to read him more. He does a great job of describing the region of Cainsmarsh and the phenomenon that is happening there. I'll give a slight spoiler and say that the thematic undertone, that man's beastly side is returning and threatens to rip civilization apart, is done with too heavy a hand in the end, though perhaps given how close Britain was to the dawn of war, subtlety was not really an appropriate approach at that time.
You should read this book for yourself. It's in the public domain and you can find it here.
I'll give you a piece of Wells' great prose to whet your appetite:
He was soon launched upon the wildest diatribe. He was transfigured by an anger that shook his feeble frame. He had fixed upon the local archæologists and naturalists as the chief objective for his tirade, but mixed up with that in the oddest and most illogical way was his detestation of the high-church practices that had been introduced by the new man at Marsh Havering. Just when this Evil was being released and rising like an exhalation from the earth, when the one supreme need of the time was religion straight and stern—'STRAIGHT AND STERN,' he repeated and shook his fingers in my face—this man must come with his vestments and images and music and mummery!