Borderline : by Janette Turner Hospital (Dutton: $15.95; 287 pp.)

At its heart, Janette Turner Hospital’s third novel “Borderline” has the question--”What if?”

What if Felicity, a beautiful art historian and gallery owner, leaves Boston for a solitary weekend at her cabin in Quebec and is forced to wait at the border while Canadian Customs gives a meat truck an unusually thorough once-over. And what if the door to the truck opens, revealing among the beef carcasses, a la British painter Francis Bacon, a group of huddled and frozen illegal aliens. And what if there is another car stopped, and in it, on his way back to Canada from a power-salesmanship conference in New York, is a comical traveling insurance salesman, Gus Kelly, a man tortured by his thousand casual infidelities to his wife, by his lapses as a Catholic and a father, and by his easy success as a salesman?

And what if their eyes meet and they give in to mischievous, unmotivated impulse, and smuggle over the borderline one of the Salvadoran refugees, a beaten, raped and half-frozen woman.

Felicity and Gus fall a long way as a result of their mutual impulse. Both become involved in a world that remains mysterious to them--the crossing of borderlines by political refugees, the consequences of politics in countries they’ve never seen. And the reader is lead back into their history, particularly Felicity’s, to be shown how the impulse fits in with the past and determines their short futures.

The novel is narrated--constructed before the reader’s eyes, really--by Felicity’s stepson, a Montreal piano tuner close to her in age, who has long been in love with her and for as long has hated his father. Felicity’s ex-husband is a famous painter, a philandering and bombastic figure who resembles the old painter in John Fowles’ “The Ebony Tower.” Reacting against his father, Jean-Marc removes himself from the world. He would never give in to an impulse and become so involved as Gus and Felicity do. “Mr. Piano Tuner to the fore. El Magico, the mathematician, transformer of discord into smooth tones. . . . I can be, when occasion calls, a master of arch savagery. I can dart in with the quip modest, the reply churlish, the reproof valiant.”

Unfortunately, Jean-Marc is the least appealing character in the novel, and in the end his reunion with his father seems a high price to pay for losing two characters who we come to like and admire. It is most often through Jean-Marc that Hospital presses too hard for meaning and tries to make the reader aware of the cleverness of her story and its interweavings.

Hospital, an Australian now living in Boston and Canada, demonstrates in “Borderline” her intelligence and strength as a novelist, and her ambitiousness. If she sometimes overreaches, at least her work is traveling in the right direction.