BOOK REVIEW / FICTION : The Diplomat and the Model and the Past That Binds Them : THE VOYAGE <i> by Robert MacNeil</i> ; Doubleday; $23.95, 304 pages

Inside every journalist is a novelist or a poet scratching and howling to get out. So goes the folklore, anyway. And when the journalist is as distinguished as Robert MacNeil, who retires this month as co-host of PBS’ ‘Mac/ Lehrer NewsHour,” we are naturally curious. What is he going to say in this, his second novel (following “Burden of Desire”), now that he can say anything he wants?

The two kinds of writing interact unpredictably. Journalism disciplined Ernest Hemingway’s style, did nothing for Theodore Dreiser’s, but gave both a wider knowledge of the world. Journalism was a chrysalis from which Tom Robbins fluttered free, a straitjacket from which John Hersey never quite escaped.

MacNeil’s case appears different from any of these. What he wants to say, apparently, is just what other popular novelists are saying. “The Voyage” is a romantic thriller about a Canadian diplomat, David Lyon, whose career is endangered by his illicit affair with a sexy super-model, Francesca D’Anielli. Almost anyone could have written it with a little research; MacNeil’s special expertise wasn’t needed.

The novel jump-cuts, in true thriller fashion, between two converging plot lines. In autumn, 1991, Francesca sails a yacht from Finland to Sweden. Meanwhile, in New York, where he is consul general, David is notified through back channels that the yacht has been found drifting, abandoned. Francesca, presumably, is dead. She has left behind a note addressed to David, containing a fragment of Baudelaire’s poem “The Voyage,” as well as indiscreet letters he has written her.

David agonizes. In only a few hours, the news will be made public. It may destroy his painfully rebuilt marriage. His wife, Marilyn, nearly left him when she learned of the affair years ago. And it surely will spoil his chances of landing a Cabinet post, which Prime Minister Brian Mulroney has hinted he will get if he accepts the Conservatives’ invitation to run for Parliament.

We flash back to David’s meeting with a 20-year-old Francesca in Guyana; their trysts in cities around the world; her burgeoning fashion career and growing instability; their failure at key moments to forge a commitment; David’s decision, having betrayed Marilyn, to stand by her, and his rise through the ranks of External Affairs (Canada’s state department) via postings from Lusaka, Zambia, to Ottawa to Helsinki.

To David, Francesca is a wind from the tropics who thaws his northern reserve and fills the sails of his creativity. To Francesca, David is the anchor she needs to halt her life’s drift.

In 1975, before these fantasies went sour, they pledged to meet 16 years later, on Francesca’s 40th birthday, in a Finnish island cabin owned by Kaarina Koivisto, her partner in a Helsinki dress shop. Before Kaarina died of breast cancer, her serene, centered life inspired Francesca to begin freeing herself from dependence on beauty and on men’s response to it. Yet here she is, on the yacht, sailing to keep her appointment with David even though she knows he won’t be there.

One problem with the converging-plots scheme is that Francesca’s voyage must dawdle while David’s career sprints to catch up. This isn’t all bad. She is the more interesting character, and MacNeil, who knows a lot about sailing, devotes his best writing to it. But when we learn, fairly late in the novel, that Francesca’s real plan is crazy, vindictive, at odds with the woman we have come to know and like, we feel that MacNeil has held out on us. He acknowledges this with a last-minute flurry of negative information about her. The climax, though full of suspense, is resolved by sheer luck.

Only now and then does MacNeil do the kind of summing up we expect of such a seasoned observer of the world. Mulroney, he notes, did “many things David considered sensible, yet managed to surround himself with an appearance of sleaze.” His liberal predecessor, Pierre Trudeau, “continually annoyed Washington to no sound purpose.” George Bush was “the most untalented politician of his time.”

A sub-theme of “The Voyage” is Canada’s long struggle to maintain some independence--political, economic and cultural--from its powerful neighbor to the south. That struggle, David mourns in what seems to be MacNeil’s true voice, may have been doomed by the North American Free Trade Agreement, which is leaving Canada unable to pay for its vaunted social services. “He used to think Canadians were the social democratic opposition to America’s reflexive conservatism. Now Canada seemed caught in the same rightist tide . . . that pitiless common market of ideas and values.”

Good journalism could be written about this, and maybe decent fiction. Here, though, it isn’t the story or even a significant part it; just a scribbling in the margins.