The first part of the film shows the police investigation, with some examples of the seedier side of Boston life, including promiscuity in the adult quarters of the city. The second part shows the apprehension of DeSalvo. The intention of Officer Bottomly and the law is to answer the question presented in the film's famous print ad:
- Why did 13 women open their doors to the Boston Strangler?
When released film critic Roger Ebert criticized the film's content, writing, "The Boston Strangler requires a judgment not only on the quality of the film (very good), but also on its moral and ethical implications...The events described in Frank's book have been altered considerably in the film. This is essentially a work of fiction 'based' on the real events. And based on them in such a way to entertain us, which it does, but for the wrong reasons, I believe. This film, which was made so well, should not have been made at all."
In the same vein, The New York Times film critic Renata Adler, wrote "The Boston Stranger represents an incredible collapse of taste, judgment, decency, prose, insight, journalism and movie technique, and yet—through certain prurient options that it does not take—it is not quite the popular exploitation film that one might think. It is as though someone had gone out to do a serious piece of reporting and come up with 4,000 clippings from a sensationalist tabloid. It has no depth, no timing, no facts of any interest and yet, without any hesitation, it uses the name and pretends to report the story of a living man, who was neither convicted nor indicted for the crimes it ascribes to him. Tony Curtis 'stars'—the program credits word—as what the movie takes to be the Boston strangler."
More recently, film critic Dennis Schwartz discussed the film's style, writing, "What mostly filled the split-screen was the many interrogation scenes, where on one side was the suspect and interrogator in the present and on the other side the suspect and his interrogator in flashbacks. Fleischer eschews the graphic violence in the murders and aims instead to try and understand the killer through the script's bogus psychology. The big things the film tried didn't pan out as that interesting, as the flashy camera work counteracts the conventional storyline chronicling the rise, manhunt, fall, and prosecution of De Salvo.