This is not really a Legion story, but it is a story which introduces a Legionnaire, so it’s included in the chronology. The Legionnaire in question is Mon-El, otherwise known as Lar Gand of the planet Daxam. Daxam is apparently similar, environmentally, to Krypton, and so its inhabitants are super-powered on Earth, just as Superboy is.
I have no idea if Mon-El was created with Legion membership in mind. It seems unlikely for a couple of reasons. One, why would he be needed on a team that already includes Superboy and Supergirl? While the Legion had yet to establish a “no duplication of powers” rule, as they later would (and they would play fast and loose with it even then!), it still doesn’t make much sense to add a pseudo-Kryptonian to the lineup. Also, Mon-El gives his age as “at least 18,” which might make him too old for membership.
At any rate, the story begins with Superboy rescuing a one-man spaceship before it crashes to Earth. Finding its sole occupant, Superboy immediately goes through his pockets. (I mean, it’s what Batman would do, but Batman didn’t exist yet!) He finds a star chart written in his father, Jor-El’s handwriting. He then uses X-Ray vision to check under the boy’s clothes, where there is a pendant of “Kryptonian metal” inscribed “with love” by Jor-El and Lara. Okay, to be fair, he was probably using his X-Ray vision to check for internal injuries, but, after the going-through-the-pockets trick, he’s a bit suspect.
Because the star-chart says “my son’s rocket,” Superboy jumps to the conclusion that this kid, who has amnesia, must be his older brother, who did not age during the past 17 years because he was thrown into suspended animation by the force of the destruction of Krypton. Never is it explained how a planet blowing up next to one puts one in suspended animation, but okay. Superboy names his new “brother” Mon-El, because he found him on a Monday.
Superboy tells Mon-El the story of his infancy and the destruction of Krypton. He learned this, he says, by “overtaking light rays from the past.” (Which he did, in Superman #146, July, 1961.) As a result of that trip, he knows what his parents looked like and how they died. One wonders why he does not also know that he did not have a big brother.
When Mon-El asks if he isn’t a bit old to be the child of Jor-El and Lara, Superboy describes a theory that Kryptonians aged really fast. One wonders if Mon-El’s X-Ray vision spotted that theory in Superboy’s alimentary canal before he voiced it, ’cause it certainly came right out of someone’s ass.
As they get undressed to go to bed, Clark notices that Mon-El’s uniform belt is “not Kryptonian metal,” and gets suspicious. When Mon-El falls asleep, Clark retrieves some green Kryptonite and exposes Mon-El to it, to see if he’s really Kryptonian. Turns out he’s not. Superboy’s response? “That Phoney!” [sic]. When they wake up the next morning and Mon-El begins talking about their plans for the day, Clark angrily reflects that he’s just “Keeping up the pretense.” Even the text box that ends the story on this cliffhanger (although part two starts on the next page) asks, “Who is the cunning stranger?”
Okay, hold up there, Sparky. Never at any time did Mon-El claim to be Superboy’s brother. That was a conclusion that Clark jumped to faster than he could leap a tall building in a single bound. So Clark built up expectations based on circumstantial evidence, and it’s Mon-El’s fault? The poor guy has amnesia! His ship crashed, and he was lying there, minding his own business while some kid looked for his wallet, perved on him with X-Ray vision, and then said, “S’up, Bro?” How is this Mon-El’s fault?
I can’t call the turn of events unrealistic, though. Irrational as he’s being here, Superboy is illustrating a sad fact of human behavior: when we decide that another person is someone they’re not, and get all wrapped up in the fantasy, we usually wind up blaming that person for just being who they were all along.
Which goes to show that Clark Kent, at one level, is a perfectly normal human being. And, in the words of playwright Mary Chase, “You know what bastards they are!”