Her code name was "Parlor Maid." She was a spy whose information about China found its way to four American presidents. For 20 years Katrina Leung's handler was a freewheeling FBI agent named J.J. Smith. But "Parlor Maid" and J.J. were more than agent and asset: they were lovers. Last April, the government claimed "Parlor Maid" was a double agent--spying for China with the help of her lover J.J.--and the two were arrested. FRONTLINE investigates a story of sex, secrets, risk, patriotism and power, and explores how U.S. intelligence about China has been seriously compromised.