Save Me Again

When Spencer Reid finds himself in the grasp of a killer, his life flashing before him, he tries to think of a way to fight back, a way to keep himself from dying. He hears the click, and squeezes his eyes shut as the gun is jammed against his ribcage, and winces as the trigger is pulled. All he hears is the loud 'bang' of the gun going off again, and the shouting of a female, who identifies herself another FBI agent, but he does recognize it. Who is she? And why is she here at the exact moment that he needs someone?