By unfortunate happenstance, Gail Walker is driving with her daughter Sarah in Portland when they blunder into a crime scene in which an informant is killed by the thug Hayden Stalzy. Gail is seriously wounded, and both she and her daughter are witnesses to Stazy's shooting of the informant. Stalzy has an ugly, distinguishing mark of a scar on his face, so he is not difficult to ID. The film unfolds the harrowing story of little Sarah protected from Stalzy by a kind female detective while her mother lies in critical condition in hospital.

The maternal instinct of the film's title is emotional reaction of Detective Heather Parker, who takes young Sarah under her wing. They receive support from Detective Travis Lancaster, and a romantic intrigue unfolds between the cowboy Travis and Heather. A risky visit to the hospital for Sarah to see her mom places Heather's career in jeopardy, yet defines her humanity. In a number of scenes with solid fight choreography, Heather proves herself to be Wonder Woman.

An especially interesting character was the neighborhood gossip, Lois, who befriended little Sarah and offered her fudge. Unfortunately, Sarah also allowed Sarah to use her computer, which allowed Scarface to track their whereabouts. One of the most moving scenes of the film was Lois's resolve in refusing to reveal to Stalzy the whereabouts of Sarah and Heather.

The most touching part of the film was the bonding of the detective, who admits that she doesn't like children, and the youngster. The performances were first-rate, and there was an effective denouement in which the hard-as-nails Captain Nichols was able to exonerate Detective Parker and give his blessing for her budding romance with the cowboy.