A pair of lovers find a dead man beside his car, with a cell phone in his mouth and the alarm going off.
The BAU determines that the first victim was Douglas Clark, that the clock alarm on the cell phone in his mouth was on, and was set for midnight. He was stabbed at least 12 times, and he was gay.
A second man - Kevin Brubaker - is killed. Tires are punctured on the victim's car and all of the windows are broken. He too has a cell phone in his mouth, with an alarm set for midnight.
Hotch interviews Clark's gay partner who tells him that he still in the closet. He didn't want anyone to know.
Kate shows Hotch some video on Brubaker's phone. He had videotaped his straight sexual liaisons without his partners' knowledge. There are four videos in all with different partners.
The medical examiner tells JJ and Morgan that both victims had distinct puncture wounds in their necks.
JJ figures out that the puncture wounds could have been caused by a stiletto heel, and that the Unsub is a woman.
Kate interviews one of the recipients of the porn video sent by Brubaker, and convinces him to cooperate with the investigation by convincing him that the videos were of underage girls, making his actions complicit in the distribution of child porn. After the interview is done, Morgan tells her the women were of age, not underage. Kate had deceived the guy in order to obtain his help.
The Unsub meets a third man, takes him back to her place and ties him up in the bed. Then she puts ashes on her face, comes back into the room and stabs him to death.
Rossi and Morgan find the third victim with a watch in his mouth - the alarm is set to midnight.
The BAU sets the profile. They're looking for a woman who is a female annihilator - a black widow who lures men in with a promise of sex, then kills them. They say she is likely in her twenties and attractive. She probably has little money. She puts a cell phone or watch into her victims' mouths, which is her way of saying "time's up" or "a new day dawns, but not for you". The triggering event might be an assault that she suffered, so she appointed herself judge, jury and executioner, killing surrogates for her own attacker.
The Unsub returns to her job at a dry cleaner's only to be told that her pay was being docked by $20 because she had been late five times during the month.
JJ figures out that the Unsub is living out a fantasy as a modern day Cinderella. Her victims were Prince Charmings who didn't live up to her fantasies.
The dry cleaner owner figures out that the Unsub - Claire - has been stealing some of his customer's clothes and tells his son that she's fired. His son offers to give her the news personally.
The Unsub shows up at a bar, where yet another guy takes an interest in her. She dances with him and then leaves with him. When it turns out that he was just interested in sex, she gets out of the car, uses a rock to knock him senseless then kills him with one of her stiletto shoes.
The dry cleaner's son shows up at the Unsub's house the next day to give her the bad news about her job. She's on her way to the cemetery to put some flowers at her father's grave, so he offers to drive her there.
Kate, Morgan and Rossi figure out that the Unsub works and steals her clothing at a dry cleaning company. And that she likely suffered a childhood trauma, such as incest. It turns out that was exactly the case.
While sitting with her at her father's grave, the dry cleaner's son realizes that the grave actually belongs to a woman who died 100 years ago. This conflicts with the Unsub's belief, so she gets angry at him. When he won't relent, she picks up a flower pot and hits him over the head with it.
Claire Rawley is the Unsub. Garcia gives Morgan her address.
Hotch and Kate show up at Rawley's house to find she isn't home. They find out that the dry cleaner's son is with her, so that track his cellphone to the cemetery.
Reid and JJ show up at the cemetery. As JJ trains her gun on her, Reid pretends to be Prince Charming, and acts out the final scene in the Cinderella story, placing her shoe on her foot and kissing her hand.