"The Mystery of 31 New Inn" by R. Austin Freeman is a detective novel written in the early 20th century. The story revolves around Dr. Jervis, a medical doctor who becomes involved in a perplexing case concerning a mysterious patient named Mr. Graves. Mr. Graves appears to be gravely ill under unusual circumstances that suggest he may have been poisoned.

At the beginning of the narrative, Dr. Jervis reflects on his connection with the influential detective John Thorndyke while recounting a recent incident in his practice. He receives a call from a messenger summoning him to attend to Mr. Graves, a patient who has refused medical help until this point. Upon arriving at a dimly lit house, he is greeted by an anxious housekeeper.

When Dr. Jervis confronts Mr. Graves, he notices alarming signs indicative of opium poisoning, which leads him into a web of secrecy and uncertainty. The strange atmosphere, the housekeeper's behavior, and the ominous presence of Mr. Weiss create a tense dynamic. This raises questions about the true nature of Mr. Graves's illness, the motivations of those involved, and the dark secrets they might be hiding.