On the surface of the Moon, the real Supergirl and her evil doppelgänger square off against one another. Dark Supergirltaunts Kara, claiming her father Zor-El hated his brother and sent her to kill her own cousin. Supergirl retorts she’s just lying and pounces. As they trade blows and Dark Supergirl keeps taunting Kara, the JLA turn up, regrouped and ready to fight, but Dark Supergirl flies off into space. Supergirl tells Martian Manhunter to read her mind and “warn him they’re coming” before pursuing.
Supergirl and Dark Supergirl clash in space. As they carry on attacking one another, Supergirl lures her evil duplicate into following her to Gotham City. Both girls crash-lands on Gotham Harbor where Batman is already waiting them. Dark Supergirl scoffs at Batman and tries to kill him, but Superman and Wonder Woman arrive to help Batman out.
The evil Supergirl power-dives into the real one and switches their costumes at super-speed. Now the Leaguers have no way of knowing which one is the real Supergirl and which one is the duplicate. Superman attacks both of them, thinking the real Supergirl will not defend herself, but both girls strike back. Wonder Woman manages to subdue Kara while Superman clashes with Dark Supergirl -still dressed as Supergirl-. Dark Supergirl hits him, recalling him everyone tells she’s stronger. Superman hits her back and replies she shouldn’t believe everything she hears because he’s always instinctively holding back and he’s sure he can stop her whenever he wants. Dark Supergirl invites him to test his theory, but Batman separates them with his Kryptonite Ring. Suddenly Supergirl breaks in and states there’s one way to end this: Wonder Woman’s Lasso of Truth.
Wonder Woman binds them together and commands the Supergirls to tell her who is the real Kara Zor-El. Since the mystical properties of the lasso can not adequately distinguish between the two, it merges them back together and Supergirl is once again whole.
Superman hugs his cousin. Kara states she’s still missing most of her memories but it doesn’t matter: the three of them are her surrogate family now and her mentors from now on. Later Supergirl flies around freely, thinking her story is just beginning, and what it’s up to her finding out her true self… but she knows she’s Kara Zor-El, Supergirl, and that’s what truly matters.