The Siamese Twin Mystery is an English language American novel written in 1933 by Ellery Queen.

[1] Inspector Queen and his son Ellery are vacationing in a mountainous area of the United States when they are forced by a forest fire to seek shelter at Arrow Head, the mountaintop home of famous surgeon Dr. John Xavier, which soon becomes impossible to escape due to the fire.

There the sleuths meet an unusual assortment of characters, beginning with Dr. Xavier and his wife Sarah; his brother Mark; his medical assistant Percival Holmes; his handyman "Bones"; the housekeeper, Mrs. Wheary; and a houseguest, Ann Forrest.

That night, Ellery spies a strange crab-like shape moving in the upstairs hallway, and, independently, Inspector Queen discovers that also present at the house is the wealthy Marie Carreau, a fixture of Washington high society; Ann is Mrs. Carreau's personal secretary.

The previous night, the Queens had encountered on the road a heavyset and unsavory man, who — his escape blocked by the fire — now returns to the house, giving his name unconvincingly as "Smith."

It is further revealed that Marie Carreau had come to the house to consult with Dr. Xavier on the subject of her teenage sons, Julian and Francis, who are xiphopagous conjoined twins.

Mark, in a panic, tries to escape; Inspector Queen shoots him and returns him to the house, gravely wounded.

Further, Ellery observes that the French name for the suit of diamonds is carreaux, and the image on a jack is that of two conjoined young men!

An airplane drops a message from the local sheriff, informing the party that all attempts to contain the fire have failed and advising them to dig firebreaks around the house.

The character of Ellery Queen and the more-or-less locked room mystery were probably initially suggested by the novels featuring detective Philo Vance by S.S. Van Dine, which were very popular at the time.

Other details of the lives of the fictional Queen family contained in earlier introductions have now disappeared and are never mentioned again; the introductory device of "J.J.

The first paperback edition of this novel does not contain such a challenge, possibly because World War II paper restrictions in the United States meant that all unessential material must be trimmed.