Mark Twain's Mysterious Stranger is the final novel the author worked on, but never completed. Between 1897 and 1908, Twain wrote several versions of this book about Satan which addressed Twain's views regarding the human race and his ideas concerning morals. Various titles were given to these attempts, but the subject was always reflective of Twain's difficulty with understanding the creation and purpose of man.

Not only have several versions of the book surfaced, but so has controversy over its authenticity as a Mark Twain work. Albert Bigelow Paine, as the literary executor of Twain's estate, received sole proprietorship of Mark Twain's unfinished works following Twain's death. In 1916, Paine released The Mysterious Stranger calling it the last major work of Mark Twain. In 1963, John S. Tuckey, in collaboration with other literary researchers, performed a very detailed review of Mark Twain's manuscripts for this ever-evolving book. They discovered that Paine had not only combined three of Twain's unfinished works into this released version of The Mysterious Stranger, he had also added several passages that were not the writings of Mark Twain. Thus, The Mysterious Stranger was found to be a fraud, almost forty years after Paine had published it.

The University of California Press, in 1969, published all of Twain's original manuscripts for the many versions of this book that he had worked on between 1897 and 1908. The release of the papers can be found under the title Mark Twain's Mysterious Stranger Manuscripts, which were edited by William M. Gibson. What is now considered to be the only authentic version of the book by Mark Twain, currently titled No.44, The Mysterious Stranger, was released in 1982 by the University of California Press.

A synopsis of the four versions by Mark Twain follows and are based on a compilation of data from various sources:

• St. Petersburg Fragment: Written by Mark Twain in September 1897, this version was set in St. Petersburg, the fictional town that represented Hannibal, Missouri. This manuscript was a mere nineteen-page document.
• The Chronicle of Young Satan, probably written in September of 1897, is set in medieval Austria during 1702. This version is the first large work by Mark Twain that would eventually be incorporated into St. Petersburg Fragment by Albert Paine. The main character in this work is Satan, a nephew of Satan as depicted in the Bible. The young, sinless Satan resides in Eseldorf, a village in Austria, which ironically means "assville or donkeytown" in German. Twain abruptly stopped writing this version in the middle of a scene in which Satan was entertaining a young prince from India. It is unknown why Twain stopped so suddenly.
• Schoolhouse Hill: Consisting of only 16,000 words, this version by Twain was first outlined in Twain's Notebook of 1868, wherein his notes indicate his plan to include some of his other characters and personal experiences in this novel about Satan. It is also the first time Twain uses the number 44 in relation to a character or title. Per Bernard deVoto, and editor of Twain's work and papers, the Notebook entry begins:

-Story of Little Satan, jr. [also known as -44] who came to

{Petersburg (Hannibal)}, went to school, was popular and

greatly liked by {Huck and Tom} who knew his secret...

• No. 44, The Mysterious Stranger: This version, which is also known as the "Print Shop" was written between 1902 and 1908. Set during 1490 in Austria, the story takes place shortly after the invention of printing, possibly linked to Twain's experience in the printing industry when he was younger. It is a manuscript consisting of 530 pages.
• "No. 44, The Mysterious Stranger," also known as the "Print Shop" version written between 1902 and 1908 and set in Austria in 1490, not long after the invention of printing. Several theories have been offered regarding Twain's use of "44" with regard to Satan. Some hold that the number represents Satan as an addition to the "trinity," which would then become a quadrant (or four); others feel that the use of this number regards a belief in a "quasi-god" that existed prior to Adam and Eve.

Mark Twain's Mysterious Stranger has an origin and message that is almost as strange as the evolution of this poplar work.