Captain Marvel Junior, the World's Mightiest Boy, had his origin in Whiz Comics #25 (published November 28, 1941). But to fully understand his origin, we need to take a quick look at Master Comics #21 (published November 5, 1941). Master Comics #21 tells us the origin of the villain Captain Nazi and how he was sent to the US by Hitler himself to challenge and destroy Bulletman, Captain Marvel and other heroes. Captain Nazi's plans are thwarted by both Bulletman and Captain Marvel, and at the end of the story he leaves Captain Marvel with a menacing message - Captain Nazi will be going to Whiz Comics to beat Captain Marvel there!
We pick up Captain Nazi's trail of destruction that started in Master Comics #21 in the pages of Whiz Comics #25. Trying to kill Captain Marvel's friend Whitey and trying to destroy a dam/power plant are among Nazi's dastardly deeds. Captain Marvel finally catches up to Nazi as he tries to kill a pilot flying an experimental plane. The world's mightiest mortal sends Captain Nazi flying into the waters of the bay with a tremendous punch.
Freddy Freeman and his grandfather are fishing in the bay when Freddy notices someone (Nazi) has fallen into the water and they take off to rescue him. After rescuing Captain Nazi, Nazi repays their kindness by killing Freddy's grandfather and mortally wounds Freddy. Captain Marvel rescues Freddy from the water and flies him to a hospital. Cap transforms back to Billy Batson and waits for Freddy's diagnosis. Unfortunately, the news is grim. Captain Nazi broke Freddy's back and he's not expected to survive the night.
Billy transforms to Captain Marvel, and then Freddy wakes up to see his hero and says, "Why it - it's Captain Marvel!" As Freddy says his heroes name, magic lightning strikes and he is immediately transformed into Captain Marvel Junior! Junior has all the powers of Captain Marvel, and Cap tells him he must use them to fight the forces of evil. He also tells him, "You will have to go through life in your own form, but whenever you need me, speak my name and as Captain Marvel Jr. you will be able to do the things I do..." Captain Marvel sends Junior to Master Comics to take care of Captain Nazi there.
Freddy wakes up feeling like it has all been a dream. While he has been saved from a sure death by Captain Marvel, he is destined to have a crippled leg for the rest of his life. Freddy vows to carry on and to catch Captain Nazi and put him away so he cannot hurt anyone anymore. Captain Marvel Junior would next appear in Master Comics #22 (December 5, 1941) and would also get his very own comic.
Freddy Freeman was rescuing a stranger from death and it cost the life of his grandfather and almost cost Freddy his life. Thankfully he was saved by Captain Marvel and was given the powers of Captain Marvel to help Cap in his fight on evil. That is the amazing origin of Captain Marvel Junior, the world's mightiest boy!