This issue concludes the brief 3-issue run CotU had as a reprint book in 1973 with the story from SHOWCASE #11 (1957), though re-coloured to feature the groups later red and yellow jumpsuits instead of the classic purple. The aliens were also changed from orange to green.
In this story the Challs are sent down to find some missing scientists in Antarctica. While there they find the scientists in the clutches of an alien invasion force, the Tyrans, who plan some major explosions to reduce the gravity of the planet more to their liking, as well as destroy human civilization making us ripe for conquest. Some great artwork in the underground alien lair, and later on there's a great huge alien machine which comes out of the ocean.
Bruno Premiani, best known as the co-creator of the Doom Patrol and his long stint on Tomahawk, inks the 24-page story. This might be his only time inking Kirby, unless he did some while working for S&K at Crestwood (the Kirby Checklist lists him on SHOWCASE #12 as well, but it looks quite different and I think DC's recent George Klein credit seems more likely). I really like his work on this, reminding me a lot of the texture that Wally Wood brought to later stories but not nearly as overwhelming. This might be my favourite inking in a Challengers story, which had a lot of good inking. It's also interesting that this seems to be Premiani's only DC credit in a several year period when he apparently wasn't even living in the United States. Was this done well before it was published? Or was it just a job Premiani picked up while visiting from Argentina?
This story is available in CHALLENGERS OF THE UNKNOWN ARCHIVES v1.