Edition published 1973
Though the ending of Skylark of Valeron seemed to beg for a sequel, Smith did not get around to writing one for many years. Not until 1965 did Skylark Duquesne appear in Worlds of IF Science Fiction.
The narrative of this book is more complex than that of the previous Skylark books, and tells several stories simultaneously:
The release of DuQuesne from his extradimensional prison,
The fate of the Fenachrone (not all destroyed in Skylark Three),
The battle between the Llurdi and the Jelmani, and
The resolution of the ultimate, Chloran threat to humanity.
The Llurdi are a monstrous, purely logical race that enslaves humanoid races like the Jelmani. DuQuesne, given back his physical form, encounters them and begs Seaton and Crane for help. Simultaneously, the Llurdi experiment with freeing a group of Jelmani to see if the Jelmani can, while free, develop any interesting technology. They do and DuQuesne gets it from them. Meanwhile, Seaton and Crane discover that the Chlorans are a much more dangerous threat to the universe and set about discovering the technology to defeat them.