Roger Hoskyn and Dorothy Woodcote are spending their Easter weekend holiday on a walking tour of the English countryside. Toward day's end, the pair find themselves still a long distance from a train station and decide to stop at Whiteledge, a country manor house, to ask directions. To their surprise, the butler ushers them in and leads them to bedrooms and baths, where the travellers are to wash and then join the house for dinner. Arriving at table, Roger and Dorothy are soon in the company of some interesting persons: they recognize Claudia Denbies, a striking redhead and a celebrated violinist, and master George Merrow, whose birthday the party is celebrating, as two of three horse riders who passed them that afternoon. Dorothy notices that the third rider, a tall, handsome man, is absent from the group, and an enquiry reveals that Mr. Harry Lingfield went out riding and has not yet returned. Dorothy also learns that her and Roger's invitations were given by the superstitious and eccentric Lady Catherine Leith, who didn't want Lingfield's appearance to create thirteen at table. To Roger's right sits an unsettling old lady named Mrs. Bradley, whose black eyes take in the details of all her dining companions.
Mr. Lingfield still hasn't returned to Whiteledge by the following day, and Mrs. Bradley and the travellers, under the auspices of taking the dog for a walk, retrace the trail on which Dorothy and Roger saw the horse riders. The dog disappears into a copse, and its pursuers come across the naked and headless body of a man. It appears to be the body of the missing Mr. Lingfield, but there is some opposition to this theory, notably from Claudia Denbies, Lingfield's lover. The corpse's head is not found at the scene, and an inquest only seems to raise more questions. Mrs. Bradley has been acting as a consulting psychiatrist for someone in the house, but will not reveal her patient's identity to the police. A Scottish train conductor testifies to seeing a decapitated body laying across the tracks. The inspector's suspicion falls on Mrs. Denbies, but then who is responsible for three attempts on Roger's life, and why? When a second beheading occurs, Mrs. Bradley steps in and offers a solution which incorporates archery, sculpture, second sight, seven and sixpence, the "mount of Venus," tripwire, barbed wire, and a burned-out car in high-spirited—if not entirely believable—fashion.
