The Dancing Druids (Mrs. Bradley Series 21)

Mrs. Bradley is at her devastating best, saurian perhaps but very human.
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Mike O'Hara is a young, handsome distance runner, working with his fellow athletes to catch his cousin, Gerry Gascoyne, in a cross-country game of hare and hounds. Playing a hunch, Mike separates from the group and soon encounters a man who tells him his quarry travelled down a lonely footpath. Instinct tells him otherwise, but Mike follows the path and is soon in unfamiliar territory.
As night falls and it begins to rain, Mike stops at a solitary cottage to ask for directions. The woman at the house tells Mike she's with a very sick man who has to be taken to hospital. Mike offers to help, and is soon working with a tall stranger to move the sick man--who is bundled from head to toe in blankets--onto a makeshift stretcher and into a car. Told to hold their bundled passenger upright, Mike grows more and more uneasy of the situation and increasingly alarmed at the deathly stillness of the sick man. Uncertain of what lay ahead, Mike jumps out of the moving car and tumbles out, escaping from the strange scene.
O'Hara and Gascoyne decide to tell Mike's story to Ferdinand Lestrange. The lawyer is not available, but much to the boys' luck, Ferdinand's mother is quite interested in Mike's tale. Mrs. Beatrice Adela Lestrange Bradley is on the case, and what criminal activity she uncovers--with the help of Mike and Gerry, secretary Laura Menzies, chauffeur George, nephew Denis Bradley, and a troupe of hired film extras--centers around a circle of nine prehistoric stones called the Dancing Druids.