Superman finds the diary of his father. Playing it back in the Fortress of Solitude, he uncovers the story of Jor-El's youth, education, early employment with Krypton's space agency, and his fledgling romance with astronaut cadet Lara Lor-Van.
After a tortured application process for a marriage license, the story ends with the arrival of an adult Kal-El to witness the wedding of Jor-El and Lara . . .
The how and why of Kal-El's arrival on Krypton is underplayed so much in this story that it's not very clear at all what's going on. Readers of Superman (1939) #141, however, will realize that it is a fact of Earth-1 continuity that Superman did indeed witness his biological parents' wedding.