In The Lost Girls of Willowbrook, the mystery plot hinges on mistaken identity, the difference between surface impressions and reality, and the willful manipulation of persona and image. Sage’s identical twinship results in her captivity in Willowbrook. Externally, Willowbrook presents as a peaceful place of safety for patients, while in reality they are profoundly mistreated and abused. Finally, Eddie acts as a friendly and genuinely concerned member of Willowbrook staff, but is revealed to be an unrepentant serial killer. In all three examples, people in positions of authority take shortcuts to present the image they’d rather not work hard to actually achieve.

Willowbrook authorities feel certain that Sage is actually her identical twin sister Rosemary, a patient who has recently gone missing. This case of mistaken identity is compounded by Rosemary’s propensity for occasionally claiming to be Sage, and by the fact that the twins’ parents did not inform Willowbrook that Rosemary had a sibling. Sage searches for ways to prove that she is not her sister, but Willowbrook workers refuse to believe that there is anything to her inner life beneath her exterior. Moreover, misidentifying Sage as Rosemary absolves the institution of having to figure out what actually happened to their missing patient.