The NIH team hurtles toward St. Louis when four middle-school teens fall ill to encephalitis. Meanwhile, after Connor and Natalie conduct inconclusive tests, they shock the parents when they ask for a brain biopsy to detect if the problem resulted from a sexually transmitted disease. Eva investigates a different path when a friend suggests that the girls were poisoned by a jealous rival -- even as new cases are reported. In addition, Natalie feels helpless to provide comfort for a long-suffering patient (guest star Shea Whigham) who is participating in a clinical trial.

When vacationing guests at a tropical family resort began dropping due to respiratory problems, Dr. Connor choppers in to assess the fast-spreading disease that has many contrary symptoms, forcing him to quarantine the entire island -- a bold decision that spells bad publicity for the anxious hotel company. Meanwhile, Dr. Durant stays behind in nearby Miami to run extensive lab tests and help the desperate Connor narrow the odds of finding the source of the deadly contagion.