Throughout the plot of the 1957 movie “I Was a Teenage Werewolf” Sergeant Donovan shows himself to be the movie’s moral compass of the story. This trope is used to further the plot, a foil for the story’s villain, and as a way to drive home the moral of the story.
We first encounter the Sergeant as he interrupts a fight between Tony, the main character, and Jimmy. He’s instantly set up as his role of moral compass by helping the two boys talk things through and apologize to each other. He finds a way to support both teenagers and expresses concern for the emotional state of Tony, who is constantly losing his temper. He then starts the plot rolling, as it’s his suggestion for Tony to see Dr. Brandon that leads to the boy’s transformation into a werewolf. This transformation is different from the depictions of wolves in stories like "Little Red Riding Hood ", where the wolf is a sentient and sinister creature (Perrault). Tony is instead depicted as mindless animal, which is exactly what the doctor intended.
Sergeant Donovan acts as foil to Dr. Brandon despite being the one who put Tony into the psychologist’s clutches. While the doctor is someone who tampers with emotions and chemicals, the Sergeant maintains peace and order. Donovan is also acting from a selfless angle in his attempts to help Tony, while Brandon is simply manipulating Tony for his own selfish reasons. And while Brandon is the creator of the monster, it’s Donovan who puts him down.
The final scene of “I Was a Teenage Werewolf ” also put the Sergeant in his final role for the movie; the purveyor of the moral. Tony has killed four people, including his creator, and been shot down by Donovan in an act of necessity. Sergeant Donovan then, looking at the now normal features of Tony’s face, issues the parting statement, “It's not for man to interfere in the ways of God.” Brandon sought to return Tony to a primal place, and it led to both his and Tony’s death.