That Affair Next Door focuses on a mysterious murder that has occurred in a quiet neighborhood, incidentally in the house next door to the home of the curious middle-aged amateur sleuth Miss Butterworth. One night around midnight Miss Butterworth sees a man and woman enter the Van Burnam mansion, which is supposed to be empty. The man leaves soon after, but the woman does not. On the following day, Miss Butterworth and a policeman find the body of a woman crushed to death under a cabinet in the parlor. There are way too many coincidences to be realistic, but the competitive sleuthing of Miss Butterworth and Detective Gryce keep the story moving along with seemingly definitive evidence pulling the reader to believe one person after another must be the actual murderer.