Cover art for book #1 The Alien Factor
The Ultra Thriller series is a crossover series published between August 1992 and June 1993 by Archway Paperbacks (an imprint of Simon & Schuster). It was a spin-off of The Hardy Boys Casefiles and the Tom Swift IV series and joined boy inventor Tom Swift with the Hardy boys. Although The Hardy Boys pseudonym, Franklin W. Dixon was used, this series was more akin to the Tom Swift IV series by Victor Appleton.
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