This documentary reveals how investigators proved ICU nurse Charles Cullen was killing patients — and how close he came to getting away with murder.

When mysterious lab results link a series of deaths at a local hospital, a heroic nurse teams up with two small-town detectives to go undercover and help capture fellow nurse and friend Charles Cullen, a man now believed to be one of the most prolific serial killers in American history. A harrowing tale of friendship, murder and betrayal, the trio's unrelenting pursuit of Cullen offers a penetrating look at America's healthcare system, which allowed him to go on killing within its walls for over a decade.