In a flashback, young, red-headed Lex Luthor (Michael Bell, adding a midwestern accent to his voice of Wonder Twin Zan) saves Superboy (Danny Dark, who did his best to sound younger, but not too well) from a Kryptonite meteor. A subsequent accident, mostly a miscalculation by Superboy on where to aim his super-breath, shatters their friendship. Not entirely an accurate adaptation of the account in the comics, long since ret-conned out.
Meanwhile, Giganta's origin is vastly different from the comics. Where she was originally posited as an enemy of Wonder Woman, the writers decided to match her with Apache Chief, and now we know why. A shaman (Stanley Ralph Ross, also the voice of Grodd) gives a young brave some magic dust, which enables him to grow to 50 feet tall. The woman who'd become Giganta steals the powder and uses it on herself.
For what it's worth, the producers had Louise Williams, otherwise the voice of Jayna, play Giganta's younger self. Seems they're finding she did more than one character after all. Unfortunately, Wikipedia's recent edits miscredit Stan Jones (Luthor) as the announcer, when it's clearly Bob Lloyd, as we previously discussed.
The episode also includes clips from 2 earlier episodes, making it clear this was the last episode, and that DC, ABC, & Hanna-Barbera were going in another direction with the franchise the next season.
Sidney Tucker, who posted this to Dailymotion after getting booted from YouTube, miscounted, as there were only 16 episodes of Challenge. As you can probably tell, Dailymotion has become available to use again, as it had been hacked months ago.