BOOK REVIEW / FICTION : Shedding Light on a City’s Dark Side : STRANGERS AT THE GATE <i> by Leonard Gross</i> ; Random House; $23, 424 pages

Plausible and unnerving, “Strangers at the Gate” uses the imminent return of Hong Kong to communist China as the springboard for a thriller set primarily in San Francisco and Vancouver, cities that have already begun to reel under the impact of an Asian influx.

A savvy young TV journalist is abducted on her way to dinner, thrown into the back seat of a car and driven to a remote section of the Presidio district, where she is forced from the car, pushed into a clump of bushes and cruelly slashed on both cheeks by a young Asian man wielding a sharp kitchen knife.

“You scream, you die,” her attacker says, leaving her bleeding on the ground after a few farewell kicks. Regaining consciousness, Maggie Winehouse staggers to a nearby house, where the horrified occupants call an ambulance.

Maggie had just completed a five-part series on Asian immigration, legal and not. Awakening after surgery in San Francisco General Hospital, she assumes that her report had outraged one or another of the Asian gangs--the infamous triads--that have become increasingly active in the Bay Area and elsewhere along the coast. Though Maggie had stopped short of naming triad bosses, she had alluded to their practice of laundering money by investing in San Francisco real estate.

The case is assigned to Zachary Tobias of the Asian squad of the Gang Task Force, a man who chose that particular criminal specialty after his father was murdered during a mugging. At that time, Tobias was an unmotivated UC Berkeley sophomore, halfheartedly majoring in business administration.

Radicalized, he switched to police science, winding up as a cop with impeccable social connections, a sizable trust fund and the brisk style of prose he employs as the narrator. Tobias is a West Coast equivalent of the upper-class Englishmen who regularly turn up in PBS mysteries, heading divisions in Scotland Yard and neglecting their significant others to pursue their duties. Knowledgeable, worldly and articulate, Tobias, because of his background, is able to comment extensively on the changing social structure of his native city; this supplies the book with a solid and somewhat rueful subtext rare in American police procedurals.

The job of finding Maggie’s assailant and the reason for the attack involves Tobias deeply in the complex machinations of the Asian underworld, a network far more subtle, complex and far-reaching than its immediate predecessors in organized crime. To no one’s great surprise, the new trend in smuggling is not drugs but people, a traffic that requires not only a higher level of cunning but a greater indifference to human life. When you consider the billions of eager customers for the smugglers, the potential profits are dizzying. As a result, triad rivalries reach an intensity that makes internecine Mafia warfare seem like an Appalachian feud.

The investigation does not proceed altogether smoothly, though the reader is encouraged to believe that everyone concerned would be as eager as Zach Tobias to rid San Francisco of the powers that are eroding the quality of life in the city. The cast of characters is extensive, and consists of so many recognizable types that “Strangers at the Gate” often reads like a roman a clef .

There is, of course, enough action, adventure and body contact along the way to satisfy fans of the genre, but the real hero of the novel is San Francisco itself; unique, beleaguered and ever-resilient.