A pharmaceutical cooperation just created a new chemo drug with minimal side effects, and are having a party to celebrate being bought out by Wayne Enterprises... So naturally, Catwoman is there to steal this fancy new drug for a client.
Selina doesn't waste much time, mingling with the crowd, getting false fingerprints, and slipping away while no one is looking. Doing what she does best, Selina makes her way through the labs with ease, taking out lights, setting up looped camera feeds, avoiding lasers, all that. Once Selina makes her way into the heart of the lab, she snatches the chemo drug, along with another one (requested to be destroyed), by her client, but something goes wrong, as one of the alarms triggers, at no fault to Selina.
A young guard makes his way into the lab, telling Selina to freeze, but... that's just not going to happen. Needing to destroy the second drug, Selina distracts the guard by throwing it in his face, but when it breaks... things get a little hairy. Turns out, the drug is some sort of steroid derived from venom, so the guard hulks out, and things go from bad to worse.
Selina ends up leading crazy huge guard who wants to kill her into the banquet, and with the huge distraction, plans on using that as her escape plan, but seeing what looks to be a child getting trampled in the mass panic, Selina's better side takes over, and she draws the guard's attention away from the innocents, long enough for him to be taken down by a whole bunch of sedatives. But that doesn't change the fact that Selina did steal the chemo drug, so she still has to make an escape.
With the escape made, time to deliver the chemo drug to the client... which just so happens to be Batman. Turns out, this was this whole scheme thought up by Batman to get the steroid prototype destroyed, and since well, you know... Bruce Wayne, Batman "theorizes" that when the company starts to work for Wayne Enterprises, the R&D department that could just recreate the steroid will be reassigned to working on the chemo drug. So yep... Batman set off the alarm, wiped the system of any record of the steroid, and if anyone notices the missing files, it'll just be blamed on the thief. Selina isn't exactly happy with being used (slapping Batman) but when she thinks about it, she sees the upside to it, and now Batman owes her, and she promises she'll collect.