Invisible death

"My husband is in grave danger and appeals to you for help. If you can see your way to answer this appeal, your answer must be as secret as possible, because his enemies must not know you have joined forces with us." Anthony Bathurst reaches Swallowcliffe Hall, summoned by Constance Whittaker, to protect her husband, Major Whittaker, from an unnamed threat. Bathurst enlists his friend Peter Daventry, a crack shot and good in a fight. One of the household suddenly drops dead, despite no one being anywhere near him. When poison is revealed to be the method of execution, Bathurst finds himself…