Baretta: That Sister Ain't No Cousin (1977)

Synopsis
The title character in this episode is Sister Barbara (Edith Diaz), a feisty and fearless nun who works in a barrio street clinic. Undercover cop Tony Baretta (Robert Blake) admires the way in which Sister Barbara is able to reform the various drug addicts in her care. Less admiring is an unknown assailant who murders one of the nun's patients -- then prepares to kill her off as well. ~ Hal Erickson, All Movie Guide