Sarah Reese left 'Chicago Med' after discovering Dr. Charles hesitated to save her father, causing her to lose trust in him.

Between a horrific serial killer and a shocking heart attack, Dr. Reese's Chicago Med exit was a gripping watch.

To say that Chicago Med’s Dr. Charles and Dr. Sarah Reese didn’t part on good terms is putting it mildly, but that’s how the former doc describes their falling out in Wednesday’s fall finale when his former protégé resurfaces.

Rachel DiPillo was a mainstay during the first three seasons of Chicago Med before leaving unexpectedly. Here's what you need to remember about her exit.