After three elderly women are found murdered in Boston—strangled and penetrated with foreign objects—the police conclude they’re dealing with a serial killer known as the Boston Strangler. As the killings cross police jurisdictions, Massachusetts Attorney General Edward W. Brooke appoints John S. Bottomly [Henry Fonda] as head of a Strangler Bureau to coordinate the investigation across multiple agencies. In the face of mounting pressure, several suspects are interrogated and released, while the city watches the case unfold with a growing sense of urgency.
Desperation prompts a controversial step: Bottomly brings in a psychic, Peter Hurkos [George Voskovec], whose profile points toward Eugene T. O'Rourke [William Hickey], a man described as severely masochistic. O’Rourke is taken in for psychiatric observation for ten days, but nothing concrete links him to the murders. Yet even as he is watched, another murder occurs, which ultimately clears him of involvement and shifts the focus back to a ruthless pattern plaguing the city.
Amid national tension during the 1963 funeral of John F. Kennedy on television, Albert DeSalvo [Tony Curtis] leaves his wife Irmgard DeSalvo [Carolyn Conwell] and their children under the pretense of work. He gains entry into the apartment of Dianne Cluny [Sally Kellerman] by posing as a plumber sent by the building supervisor. Inside, DeSalvo attacks her, tying her to her bed with rags ripped from her dress. He is startled by his own reflection in a mirror as he fights to subdue Dianne; she struggles free and bites his hand, and DeSalvo flees into the night.
He then attempts entry into another apartment, but the husband is home, and DeSalvo is spotted and apprehended by a passing police patrol. Found incompetent to stand trial for attempted breaking and entering, he is committed to a hospital for psychiatric observation. By chance, Bottomly and Det. Phil DiNatale [George Kennedy] pass DeSalvo in an elevator; they had just visited Dianne, the survivor of the earlier attack, and notice the wound on DeSalvo’s hand—evidence that tightens the connection to the Boston Strangler murders.
Conventional interrogation yields little progress, because the treating physician believes DeSalvo suffers from a split personality: a “normal” identity that fabricates memories to shield the murders committed by the other, killer identity. The treating physician, Dr. Nagy [Austin Willis], thinks that the hidden personality could be forced to confront the facts, but the resulting shock risks catatonia for DeSalvo. Bottomly opines that catatonia would be the second-best outcome to a conviction, highlighting the difficult legal and ethical balance at stake.
With the defense counsel insisting that new revelations remain inadmissible in court, Bottomly is granted one final round of interviews with DeSalvo. After several sessions, Bottomly manages to reveal the hidden personality behind the façade. The shock reverberates through DeSalvo, and he slips into a catatonic state, leaving the investigation at a precarious crossroads as the true scope of the killings becomes clearer through the intertwining of psychology, evidence, and the relentless drive to bring a serial killer to justice.
