"The Cadmus Project".
First appearance of The Guardian (post-CoIE).
First appearance of Dubbilex (post-CoIE).
First appearance of The Newsboy Legion (post-CoIE).
As Superman begins investigating a truck accident, he discovers the cause began deep below the city. Soon joined by The Guardian, Superman eventually discovers Project Cadmus and learns that Sleez has taken over the secret government facility.
Bonus back-up story: "Love's Labors... Private Lives". Dan Turpin is injured and falls for Maggie Sawyer during his recooperation, but when he suddenly asks her to marry him he gets a bigger surprise than she does!