Speedy Death — Gladys Mitchell

At the country manor house of Chaynings, the assembled guests--led by their blustery host, Alastair Bing--are ready to dine. The table boasts a respected scientist named Carstairs and an observant old psychoanalyst by the name of Bradley, but one chair remains vacant. Famed explorer Everard Mountjoy has yet to appear, and enquiries among the servants have yielded no results. Time passes and patience runs short, and Bing and a few male guests search for the absent Mountjoy. They find him in the bathtub of a second floor bathroom, dead from drowning. At least, it must be Mountjoy; and yet the body in the bath is clearly a woman's!
First Mrs Bradley story.