This issue reprints the 24-page story and cover from SHOWCASE #12 , the last of the four Challengers of the Unknown issues of the series before they got their own book. The Challs begin the story in flight in pursuit of the gang of Karnak. The criminals take refuge in the isolated island home of an archaeologist doing experiments on some ancient vials he found. As the Challengers approach, Karnak has his men drink the contents of the first vial, turning them into giants who go out and fight and are defeated by the Challs.
Two other vials release a fire monster and a sea monster, allowing Karnak to escape back to the mainland with the two remaining vials, one of which creates fifty clones of him which go on a crime wave. The Challs track down the real Karnak, who tries to escape using the final vial, which turns out just to be an antidote for the previous one.
The plot this time, especially the ending, is kind of a letdown, but along the way there are some cool things for Kirby to draw, especially the flame monster.
The art on this story seems to have been credited to every Kirby inker of the period at one time or another. DC seems to have settled on George Klein on their recent reprints.