Carol (Julianna Margulies) weighs her options as she decides to either pursue medical school or remain a nurse. Carter (Noah Wyle) wonders if he should turn in a fellow intern and former romantic rival for falsifying a patient's record. A couple who delights in concocting potentially dangerous "scenarios" plagues the ER staff. Ross (George Clooney) is determined to save the life of a two-year-old boy. And a genetically altered mouse is running loose in the hospital. This episode contains one of the most shocking and startling moments of ER's third season. ~ Hal Erickson, Rovi