When Batgirl and Supergirl investigate mysterious tremors in Gotham Bay, they dive underwater and discover... Lex Luthor! The crooked businessman is running a mining operation to dig up deep-sea Kryptonite deposits. But as the two friends battle Luthor and his drones, the green dust that fills the cavern quickly drains Supergirl's superpowers. What's worse, the unstable tunnels are at risk of total collapse! Can the girls shut down the Super-Villain's Kryptonite collection scheme before they get trapped beneath the waves?