Found dead in his Harley Street offices, the highly regarded medical specialist Dr. Mawsley has apparently given himself a fatal injection of strychnine. But he was not suicidal, it couldn't have been an accident, and no murderer visited him. During their weekly dinner party, an old friend brings the case to the attention of the eminent, if somewhat eccentric, Dr. Priestly, who has made a hobby out of the theory of criminal investigation. The problem is thoroughly laid out before the assembled company in after-dinner conversation, but no one can think of any useful suggestions on the matter that the police have not already considered. Dr. Priestly, however, is determined to prove a fourth alternative to murder, suicide and accident. And in the end, he provides a brilliant solution to this steadily baffling mystery.