After performing a few minor, undercover super-feats to help the children in Midvale Orphanage, Supergirl notices a Kryptonite meteorite in the woods, near some park rangers. The rangers signal Superman to retreat into the Fortress of Solitude until the Kryptonite can be disposed of.
This prompts Kara to begin thinking over everything she knows about the nature of Kryptonite and its effects on Kryptonians, as well as the nature of Kryptonian powers (which are the result of both the influence of Earth's weaker gravity and exposure to Earth's yellow sun.
Visiting Superman in the fortress, she gets a tour of the fortress, meeting, among others, a pair of Metal Eaters from another planet. She gets the idea of creating an antidote to Kryptonite, but Superman shoots it down. Determined to prove him wrong, she sets about developing a cure.
She returns to the Kryptonite meteorite and begins experimenting, getting as close as she can every day, pounding a post into the ground to mark her progress. She shows success, getting closer and closer. But when she tries to rescue Superman from another Kryptonite rock, it's revealed that she hasn't gained any immunity and that in fact, she was able to approach the first meteorite because it was devoured from the inside by a Metal Eater.