Circuitously commenting upon the drug culture of the 1970s, this Bonanza episode probes the dangers of the onetime "wonder drug" morphine. Future MASH regular Mike Farrell is cast as Dr. Will Agar, who is revealed to be a morphine addict after he inadvertently causes the death of a young patient during routine surgery. Also in the cast are Gary Busey as Dr. Agar's hapless victim Henry Johnson, David Huddleston as Henry's father, Melissa Murphy as Agar's wife Nancy, and Mons Kjellin (the son of director Alf Kjellen) as Agar's son Chris. Written by Stanley Roberts and Jack B. Sowards, "The Hidden Enemy" first aired on November 28, 1972.