This synopsis comes from Raquel in Alabama, who has a remarkably detailed memory of Astroboy episodes which she saw growing up in Puerto Rico:
"I remember this episode where a giant robot bird is destroying ships with the strong wind generated by his wings. He demands Astroboy to surrender or he will continue to do so. The bird was created by a mad scientist who wants to destroy Astroboy. The giant robot bird goes to Astroboy's house looking for him. I remember Astrogirl wanting to go with her brother, but their mother stops her. I don't remember very well after that, but somehow Astroboy arrives on this island, where the creator of Dunder has a cult of some kind, worshipping a god or godess. Astroboy hides by dressing up as one of the worshippers until he could find Dunder. When he finally finds him, they engage in a battle, where Astro destroys Dunder eyes, leaving him blind. Unable to continue fighting, Dunder hides with his creator in a cave, hoping he will give him his eyes back. There, he questions his creator about why he wants to destroy Astroboy. I'm not very sure about this, but I think the reason he gives him is because if Astroboy is destroyed, then he will be the creator of the most powerful robot in the whole world: Dunder. Somehow, Astroboy finds the cave and Dunder fights him again, blind and all. At the end of the episode, Dunder dies, but before, he tells Astroboy how much he admires him and how much he wishes he would have been a good robot like him."
Thanks so much Raquel for this wonderful description!!
It's worth mentioning that in this episode, Astrogirl briefly gets the ability to fly. She's given jets to defend herself against Dunder's attacks. Ultimately she has to give this up when Astro's jet power is damaged by one of his battles with Dunder.