If you love Det. Olivia Benson — and who doesn't? — this is one of the hardest episodes to get through. In "Undercover," Benson goes, well, undercover at a women's prison to investigate a guard who has been accused of sexually assaulting inmates. During her time behind bars, she gets a look at how cruel the prison system really is for inmates. She's left in a terribly vulnerable position, and the unthinkable happens.