A burglar is found shot dead in the home of Viscount Tallis, a wealthy industrialist. The man was equipped with a stethoscope, and appears to have been attempting to open a safe in the study.
Tallis himself is in the middle of the Atlantic Ocean, on his way to America on Government business. Although various members of his family are present in the house, together with guests, nobody has access to the safe - nobody even knows what it might contain...