Bernstein, Swan and Klein conclude Superman’s reign as King of Earth in Action 312 (May 1964).
Clark attempts to infiltrate Superman’s palace, disguising himself as the former hero. It’s not a bad idea, and the costume’s bulletproof nature saves him from being killed when he is challenged by two suspicious policemen. He does get critically injured though, and almost dies.
Clark is saved by Lori Lemaris, after a fortunate fall into the water. She brings him to Atlantis,and informs him that he needs surgery. Clark thinks about John Corben, and the operation that turned him into Metallo, which gets recapped for a couple of pages. Immune to kryptonite as Clark, he asks Lori to have the Atlantean doctors turn him into a Metallo.
Clark manages to approach Superman, and reveals the kryptonite in his chest. As he lays dying, Superman explains that he was not really evil. He had spotted a belligerent alien race approaching Earth, and took on the king stuff in order to scare them off.
Then, in a particularly lame ending, the effect of the red kryptonite wears off, and the two become one again. The Metallo body parts fall away. Superman convinces everyone that he wasn’t really evil, and everyone believes him.
Good story. Bad ending.