Chronicling the original incarnation of the Legion of Super-Heroes, the people they meet and the worlds they live in.
(I will say this for the bizarre honor-among-thieves behavior of the LSV in this story: it’s consistent.)
Long story short:
The stranger in the rocket doesn’t remember anything before his arrival. He has powers like Clark’s, and believes Clark’s theory about them being related when it’s explained to him, but he looks much older than Clark- “at least 18”. Clark calls him Mon-El in private, and helps set him up with a public identity as Bob Cobb, a traveling salesman.
As time goes on, Bob continues to adjust and enjoy his new life, using his powers to do good and help people like Clark does… but Krypto doesn’t recognize him. Clark investigates further in secret, having become suspicious of Bob, and finds more inconsistencies: Kryptonite doesn’t weaken Bob, and his belt is made from material not found on Krypton. The more he finds (and doesn’t share), the more he becomes suspicious when Bob doesn’t notice the inconsistencies and continues living alongside him as his brother.
Finally Clark stages an elaborate trick to find out the truth, by disguising lead cannonballs as kryptonite meteors to see if Bob will pretend weakness if he does.
It doesn’t go well.