I believe this was the last of the Imaginary Stories published in Superman; it may have been the last anywhere in Weisinger's domain. (Correction: My commenters note that there were several Imaginary Stories after this one, including in Superman). Note that rather oddly there is no cover hype about this being the 200th issue. This is somewhat strange; only a few months later DC would publish Batman's 200th ish with great fanfare.
In the story, Jor-El, Lara, and Kal-El survive the destruction of Krypton. Brainiac, who's a good guy in this story, shrinks Kryptonopolis instead of Kandor, in order to save it, but is unable to do the same for the rest of the planet:
Krypton explodes before that happens. And the element which powers his enlarging ray has also burned out, so he cannot expand Kryptonopolis on another uninhabited planet until he finds more. So the El family ages, with little brother Knor coming along a few years later to become a buddy for Kal.
Eventually Brainiac finds the expanding element, but before he can use it his ship is hit by a meteor and crashes on Earth. Fortunately, he had transferred a little of the element to Jor-El, who realizes he can enlarge one person to send out to the planet to become its protector.
And so, Kryptonopolis has a contest to see who will become Superman:
The contestants are eliminated one by one, until only Kal and Knor are left. Note in particular that this segment echoes the origin of Wonder Woman, who was selected to take Steve Trevor back to the USA in a very similar competition. In the finale, contrary to the cover scene, both men have to battle robots:
But Knor succeeds just before Kal can blind his robot, and so Earth ends up with the little brother, who goes to work for the Daily Planet as Ken Clarkson. His initial outing, foiling a jailbreak, goes successfully, but the next time around some aliens are prepared:
Fortunately Kal managed to synthesize some of the expanding element, and so he tries to rescue Knor. And when the aliens expose him to the Green K, we get a surprise:
Of course, if you're paying attention, you're probably wondering why the Green K killed Knor, but had a Red K effect on Kal. It turns out that Knor was just rendered immobile by the Kryptonite. When they both recover from the effects they send the aliens packing, and afterwards, Kal heads northward:
There is a note at the end of the story saying that just as this is Superman's 200th issue, it's the 100th anniversary of Canada being a "United Federation" (actually a Dominion).
Comments: Overall the story seems a little light, with few of the twists and turns that usually took place under Weisinger's editorship.
As noted earlier, there is not a lot of hype about the 200th issue, unlike the forthcoming Batman #200, which included a long conversation between two Bat-fans, a reprint of the first page from Detective #27 and a cover hyping the "200th Smash Issue". There is some mention of it in the letters page:
But Chet Barker's math is wrong; at that point it was over 29 years since Action #1.
