For a nunsploitation picture, boredom is a cardinal sin.
Anita Ekberg plays Sister Gertrude, a hospital matron with an unhinged psyche. She complains of intense headaches and steals to support a morphine addiction. Outside the hospital, she drinks, smokes, and has anonymous sex. During dinner, she reprimands a patient by stomping their dentures to pieces. In bed with a young nun, she orders her would-be paramour to “Say you're a dirty whore.”
Later, when patients turn up murdered, Sister Gertrude is the prime suspect.
By this point, the film has abandoned the lurid sleaze and manic energy that carried the first half. In their place, we get a convoluted scheme to replace the head physician and a flimsy mystery. Worse still, we guess the big twist, making the film's final half tedious.
The Killer Nun is trash, but not total trash, and that's a shame.