A wealthy young girl is found to have been raped and beaten in Central Park, but only a peony leaf on her clothing can direct Stella and Flack to the scene of the rape. Worse, no DNA is found from the rape kit and the victim has no memory of what happened, making the task of finding the perpetrator a real challenge. Meanwhile, a junkie is found shot to death in an alleyway, but the bullet that killed him is missing. Mac, Aiden, and Hawkes soon realize that a rat may have eaten the bullet, and finding the rat leads them to a robbery that occurred earlier that day.