Dr. Armstrong was a wealthy and successful doctor, but he had a problem with alcohol. He operated on a patient several years back while under the influence of alcohol, and something went terribly wrong during the surgery. The patient, Louisa Mary Clees died as a result of Armstrong's error. A nurse who was involved with the surgery knew that Armstrong had been drinking, but she choose not to say anything about it.
Armstrong was hired by Mr. Owen under the impression that he was to secretely take care of Mrs. Owen as she was in poor health. Armstrong brought his medical bag filled with syringes and different types of medicine. This seemed quite suspicious to the other guests who later thought he might be the murderer.