The Sinister Urge

Lt. Matt Carson and Sgt. Randy Stone arrive at a park crime scene and study the body of a young woman in underwear. They compare it to earlier park murders and begin to suspect a link to the local smut picture racket. The scene sets the investigation into a criminal underworld connecting pornography to violent crime.

In Jaffe's studio, Gloria Henderson's orders reach director Jaffe through Johnny Ryde to move the smut inventory to a safer location. Jaffe promises to relocate after finishing that day's shoot. After Johnny leaves, police raid the studio, arrest everyone, and seize the films and pictures.

Carson and Stone are berated by their superior for slow progress. Local businessman Romaine visits to question why taxpayer money is wasted on prosecuting pornography. Carson shows Romaine pictures of the murder victims and explains how the dirty-picture racket intersects with major crimes, leaving Romaine shaken.

Gloria is visited by Johnny in Jaffe's studio and receives updates on the raid. Johnny reveals that the first victim was killed by Dirk after she tried to blackmail Gloria. Dirk's enjoyment of killing and the teenage demand for bondage photos are discussed.

At Jake's Pizza Joint, a street fight erupts as rival smut peddlers clash. Dirk watches, while Jake negotiates for more smut from Janet. Police respond, seize the inventory, and arrest Jake and the two fighting men.

Gloria's house becomes the setting for watching 16mm reels with Johnny. They note that Dirk's instability is triggered by pornography and worry about losing control of him. Johnny insists he can keep the killer in check.

Dirk returns to the park, flirts with a woman, and then stabs her to death after stripping her clothes. The killing marks a dramatic escalation in his spree.

The film follows the path of Mary Smith as she cannot find legitimate work and is pulled into Gloria and Johnny's operation. She is recruited through a hostile audition and ends up at Jaffe's studio posing for cheesecake photos, with the promise of eventually entering pornography. Dirk later murders Mary in the park after finding her pictures, and the fingerprints on the photos lead the police toward him.

Dirk's murder spree draws attention, and Gloria's bosses from 'the Syndicate' visit to demand Dirk be killed to keep them from trouble. Gloria weighs the risks as the net tightens around her operation.

Gloria wants Dirk eliminated, but Johnny counters with a plan to dispatch him using a car with faulty brakes. Dirk drives off, the brakes fail, and he survives by leaping out of the vehicle before a supposed 'accident.'

Dirk makes it to Gloria's house to confront Johnny and learns that he is alive and that the Syndicate can replace Gloria. He hides outside as Gloria arrives and prepares to meet her contacts, while Johnny urges patience before a power shift.

Dirk ambushes Johnny in the backyard. Gloria returns, shoots Dirk, and mistakes him for Johnny in the darkness. She phones the police to cover the hit, but they arrest her when they discover both bodies and the evidence of the double murder.