Mandy is a 26 year old serial killer who murders couples, alongside her husband Cameron. She and her husband lure other couples to their hotel rooms with the promise of sex, but instead kill them after playing a sadistic game with them first.
Cameron will hold a gun to the man's head and give him a knife and tell him to slit his partner's throat otherwise they will make the woman suffer (rape/torture) before killing her anyway. Mandy watches and encourages the man to slit the woman's throat. The man does so out of mercy, and because the Klinefelds promise him freedom if he does it. After the woman has been killed the Klinefelds take the man outside of the room,seemingly letting him go as promised, only for Mandy to kill him by slitting his throat. They kill two couples this way in the episode, and it is unknown if they have killed this way before.
The CSI's eventually gather the evidence they need to get a search warrant for the couple's house. But when they arrive it turns out that both Mandy and Cameron have been shot dead in their own home, it is revealed in the following episode that they were killed by a vigilante working in the court.