Desperate to learn the secrets of man's immune system, the aliens begin operating on terrified human victims. Meanwhile, in the hope of uncovering new information about the invaders, Harrison launches a series of interviews with people who have had alien encounters. He is particularly drawn to Karen McKinney, a beautiful interview subject who is unable to recall the specific details of her experience.
Believing that hypnosis could trigger Karen's memory, Harrison enlists Suzanne's help in developing a psychological profile of Karen. During a revealing word association exercise, Karen herself is shocked when she blurts out the word "rape" after Suzanne mentions the word "alien."
Nearby, the invaders commandeer an ambulance and take over the bodies of two paramedics in order to perform their tests in an environment that's lower in radiation than their underground alien headquarters. Their medical disguise allows them to easily overtake new humans for their studies of the immune system.
Working closely together, the attraction between Harrison and Karen grows and they soon enter in a passionate affair. Although Norton and the team worry that his personal feelings toward Karen are clouding his judgement, Harrison assures them that he will remain objective about her case.
Placed under hypnosis by Suzanne, Karen relives the terrifying details of her alien encounter, revealing that they violated her by placing an alien probe inside of her. When Harrison later tells Karen what she said under hypnosis, she agrees to undergo a thorough exam to determine whether the invaders took something out of her body -- or left something in.
That evening, Harrison receives a frantic call from Karen asking him to join her at ther hotel. But by the time he arrives, it's too late -- the alien paramedics have taken her away in their ambulance.