Supergirl has been out of the hospital after her injuries on a case with the Outsiders. She hasn't contacted Boomer (Captain Boomerang) with whom she had a close friendship. Boomer is drinking away his sorrows at a bar telling his story to someone that bears a strong resemblance to Cassandra Cain, Batgirl.
Kara, Supergirl, has been spending more and more time with Powerboy. They have become closer and closer and more intimate, evidenced by a kiss after the couple put down an attempt on Kara by the DNAngels.
Elsewhere, in a Tibetan lair of the League of Assassins, Cassandra Cain is discussing her "tests" against Supergirl with her client. Cain plans on fulfilling the contract that night.
At Supergirl's apartment, she tells Powerboy that in order to find who's trying to kill her, she needs to go someplace on her own. Powerboy is not happy with being left behind and grabs Kara's wrist to detain her. She calms him down and leaves him with Streaky, teasing that it is the cat's inability to use her litter box that has earned her the name. Powerboy frowns as a call from Boomer comes in.
Supergirl goes to the Batcave and uses their computer while Batman is elsewhere. Robin tells her she can't just come in and use it whenever she wants, but Kara sways him by revealing that Wonder Girl revealed to her that she and the Boy Wonder shared a kiss. Supergirl asks for information on Cassandra and the League of Assassins. Robin says she's the only person to ever have hurt Bruce, which makes her the most dangerous person on the planet.
Kara sets off to confront the League, but the former Batgirl was ready for her. Using diamond dust, red wavelength uvb lights, and dressed in her Batgirl costume, Cassandra attacks Supergirl. The narrative shifts back and forth from the fight to flashbacks of her father seemingly experimenting on her, telling her that she's special.
During the fight, she sees Boomer beaten and chained overhead. Captain Boomerang is at first relieved to see that Supergirl has responded to his attempts to contact her to rescue him, but is shocked to discover Kara is ignorant of his calls. Kara is trying to explain when Cassandra takes advantage and slashes Kara's stomach with a energy katana, drawing blood. Grabbing her from behind, Cassandra and Supergirl are both surprised when some crystal-like blades protrude from Supergirl's back and stomach wound, impaling Batgirl's shoulder and side. Kara places a call to Powerboy to help her with the injured Boomer, interrupting Powerboy from adding another photo of her to his secret shrine to her.