Dr. Jervis is summoned to the aid of a dying man, but on arrival, he finds the man already dead of an opium overdose. Could the man’s death be related to will that was abruptly, and recently, changed for no obvious reason? Dr. Thorndyke is called to investigate and see if he can link the two cases.

The Mystery of 31, New Inn was written as a novella in 1905 and expanded into a novel in 1912. Dr. Jervis, a Watson-esque sidekick introduced in The Red Thumbmark , takes a prominent role.