Semi-Dual and his aides Gordon Glace and Jim Bryce battle a villain called the Blood Eagle, who is using a Hyde formula to remove inhibitions. Semi’s ally Hiwot says Robert Louis Stevenson’s account of Dr. Jekyll and Mr. Hyde was based in fact. At Semi’s request, Gordon and Jim enlist the aid of Professor Xenophon Xerxes Zapt. Semi has been corresponding with a young explorer named Croft, who will soon attempt to reach Sirius, having sensed his soulmate calling to him. Occult detective Semi-Dual (or Prince Abdul Omar, to use his real name) appeared in tales by J. U. Giesy and Junius B. Smith in The Cavalier and Argosy All-Story Weekly from 1912-1934. Professor Xenophon Xerxes Zapt was the subject of four comedic stories by Giesy in All-Story Magazine and Weird Tales from 1915-1925. Croft is Jason Croft, the protagonist of Giesy's novels Palos of the Dog Star Pack, The Mouthpiece of Zitu, and Jason, Son of Jason. Semi-Dual and Mr. Hyde are in the CU, so this novel brings in Professor Zapt and Jason Croft.
This crossover is one of hundreds covered in my book Crossovers Expanded: A Secret Chronology of the World Volume 3, which will be published by Meteor House! All three volumes are AUTHORIZED companions to Win Scott Eckert's Crossovers: A Secret Chronology of the World Volumes 1 and 2!