Dueling scenarios of life, love

and who hasn’t? -- how their life might have turned out had they pursued a different path will understand the allure of Lionel Shriver’s ninth novel.

Like the 1998 movie “Sliding Doors,” starring Gwyneth Paltrow, “The Post-Birthday World” spins two different scenarios for Shriver’s protagonist, depending not on whether she catches a train but on a decision she makes at a crucial romantic turning point in her life.

Irina McGovern, a 42-year-old American living in London, where she illustrates children’s books, comes to a fork in the road and, as the old saying goes, takes it -- or, at any rate, follows the road not taken all the way to its end, if only in her head.

She’s been together for 10 reasonably happy years with fellow American Lawrence Trainer, who works at a think tank. While Lawrence is away on business, Irina joins their friend, Ramsey Acton, for a lavish, sake-soaked sushi birthday dinner.

To her dismay, she finds herself desperately attracted to Ramsey, a South London snooker champion who is as dapper and irresponsible as Lawrence is staid and steady. (Shriver explains that snooker, which rhymes with “lucre” in its British pronunciation but “looker” in its American, is a popular, televised sport in England, a form of pool played on a large table.)

Irina realizes “with perfect certainty that she now stood at the most consequential crossroads of her life.”

In alternating chapters, Shriver meticulously explores how her heroine’s life might play out with each man. In one scheme, Irina remains faithful to emotionally remote, “chronically condescending” Lawrence. In another, she changes partners but doesn’t always waltz.

It’s an intriguing idea. In reality, a person would either have to choose between these two radically different men or try to finagle some combination. Fiction allows for richer possibilities, including parallel universes and dueling narrative strands. At first blush, this is what “The Post-Birthday World” seems to be exploring.

But there’s also a less exciting explanation for Shriver’s two-pronged narrative: Irina sticks with Lawrence in the so-called “real” story, while the version in which she runs off with Ramsey is an extended fantasy.

Late in the novel, Shriver explicitly refers to Ramsey as Irina’s “parallel-universe fancy man, The Chap She Almost Kissed,” and notes that he “had become a funny mental dependency, as if another life were running alongside this one, perhaps no better or worse but certainly different.”

Shriver’s manipulations -- which play on the reader’s willing suspension of disbelief -- raise numerous metafictional questions. All fiction is a form of fantasy, the work of an author’s imagination on a reader’s imagination. How, then, should we differentiate between figments of an author’s imagination and that of her character? Is one scenario more plausible than the other? Does the fact that the chapters in which Irina marries Ramsey always come first and aren’t altogether rosy heighten our tendency to believe them?

Shriver deliberately blurs the lines, wanting Irina’s possible world to be as convincing as her “real” world in order to explore themes of desire, guilt and fulfillment. She compares and contrasts Irina’s two scenarios down to the smallest domestic details, lending verisimilitude to both and underscoring the message of her novel’s wry epigraph, “ ‘Nobody’s perfect.’ -- Known Fact.”

Conversation with Lawrence, a terror specialist, often involves eye-glazing lectures about Northern Ireland. Ramsey’s equally numbing discourses center around snooker shots and tournaments.

Then, of course, there’s the sex. Lawrence is routine and distant in bed. Sex with Ramsey, on the other hand, is varied and addictive. But not everything is grand with Ramsey: He’s terribly possessive and jealous of Irina’s abiding affection for Lawrence. He drinks excessively and picks fights that go on for days.

Shriver’s last novel, “We Need to Talk About Kevin,” tackled a mother’s guilt over her teenage son’s murderous Columbine-like rampage at his high school. In “The Post-Birthday World,” the recent historical events she uses to frame her story -- Princess Diana’s death and the Sept. 11 attacks -- come across as handy but unearned metaphors.

Nevertheless, Shriver again demonstrates that she’s capable of patiently teasing out the ramifications of her ideas.

But doggedly pursued over 500-plus pages -- whether in Irina’s head or not -- the comparison between life with Ramsey and life with Lawrence devolves into an interminable list of pros and cons. As an alternate reality, the Ramsey scenarios pique our interest.

As Irina’s extended fantasy, they show that the poor woman has made her choice, yet can’t let go of the alternative and keeps picking over might-have-beens.

A benign effect of Shriver’s intense focus is that Irina, Lawrence and Ramsey emerge as fully realized characters with distinct voices. She nails Ramsey’s accent -- “baffroom,” “d’ya fink” -- along with Lawrence’s irritation with what he calls Irina’s faux-British locutions.

Originally from North Carolina, Shriver has lived in the United Kingdom for nearly 20 years. Occasionally, a Britishism, such as “carrier bag” for “shopping bag,” slips in inappropriately, on American soil.

Shriver has a tendency to underestimate her readers, blatantly spelling out her points. Irina not only produces a children’s book with a similar double-pronged plot, but she also outlines her theme to Ramsey -- or, rather, she imagines producing such a children’s book and explicating it to Ramsey: “The idea is that you don’t have only one destiny ... whichever direction you go, there are going to be upsides and downsides. You’re dealing with a set of trade-offs, and not one perfect course.”

The road to disappointing writing is paved with cliches, and Shriver rolls out enough to resurface a highway. Her characters “lived the life of Riley, and got away with murder, too,” “laid down the law,” “face the music,” and get on “like a house afire.”

Novels with multiple endings -- such as John Fowles’ “The French Lieutenant’s Woman” -- can leave readers dangling. Shriver, instead, deftly fuses her two-stranded narrative into a single, cohesive conclusion.

It’s an impressive technical feat that goes a long way toward redeeming some of the shortcomings of this intriguing but exhausting book.

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Heller McAlpin is a critic whose reviews have appeared in Newsday, the San Francisco Chronicle and a variety of other publications.