Lionel Shriver first gained widespread literary recognition with “We Need to Talk About Kevin,” an unsettling novel about a woman whose 15-year-old son commits a school massacre. Shriver has continued to ground her fiction in topical subjects: “The Mandibles” imagined Mexico building a wall to keep out its desperate northern neighbors, and “So Much for That” took a satiric look at one man’s struggles with the U.S. health-care system. She turns her attention to her adopted homeland in “Should We Stay or Should We Go,” which uses a middle-aged British couple’s decision to commit suicide together on the wife’s 80th birthday as a springboard to a dozen alternate outcomes. They run the gamut from blackly funny to apocalyptic, with a few surprisingly cheerful stops along the way.

Kay and Cyril Wilkinson make their pact in October 1991 at their home in Lambeth, South London. They’ve just returned from the funeral of her father, whose 14-year mental and physical deterioration has been so exhausting, says Kay, “he used up even the miserable amount of energy we’d need to celebrate the fact that he’s dead at last.” Readers of Shriver’s previous work will recognize the bluntly unsentimental attitude, and the smooth way she inserts essential details into the couple’s post-funeral conversation. Kay is 51, Cyril 52; he’s an idealistic socialist, she’s more pragmatic and conservative. They both work for the National Health Service — she’s a nurse, he’s a GP — so they see every day the consequences of lives extended beyond people’s ability to care for themselves. They don’t seem especially attached to their three children: self-dramatizing Hayley, self-serving Roy and dutiful Simon. Suicide is Cyril’s idea, which will prove significant later, but Kay agrees, and she’s no pushover — which will also prove significant.

Shriver quickly sketches the couple’s trajectory across three decades before the appointed day in 2020, planting plot seeds that will flower in multiple ways. After retirement, they draw on their house’s equity to take a series of exotic vacations thoroughly enjoyed by Kay and tolerated by Cyril, who finds old age “less than enchanting.” The referendum on Brexit, which he adamantly opposes, hardens his resolve to leave this unsatisfactory world. But Kay (who covertly votes Leave) grows increasingly panicky. She compensates by obsessively planning their memorial service and making a month’s worth of farewell dates with friends and family, all canceled as the covid-19 pandemic arrives just a few weeks before Kay’s landmark birthday. In Chapter 2, “The First Last Supper,” she slips off to the bathroom during their supposed final meal and impulsively sends a text message: “Cyril had claimed that they were making a ‘calculated gamble,’ and in the spirit of a poker game Kay was introducing a wild card.”

It’s an apt metaphor. Shriver deals Kay and Cyril a series of hands to play out, introducing new variables each time but maintaining the rules of the game set by their personalities and the state of the world around them. In several versions, Hayley bursts into the house after receiving Kay’s text, usually followed by an emergency medical team. Consequences range from a scolding to a grim, two-chapter confinement in a horrendous “care-home,” the British term for nursing home and in Shriver’s sardonic presentation as bad as any place at the bottom of America’s corporate health-care system. Just over half of the scenarios are equally dark; they include dystopian futures involving an out-of-control refugee crisis and a miracle drug that leads to worldwide anomie. It begins to seem preferable to be killed by “the archetypal White Van Man,” a stand-in for fate who runs down one or both of the Wilkinsons in several different chapters — though this running gag would be funnier to American readers if the author had bothered to explain that it’s Britspeak for a loudmouthed, low-class lout.

Mercifully, Shriver also offers Cyril and Kay a few happier possible futures, though it’s probably telling that she titles the most optimistic chapter “Once Upon a Time in Lambeth.” “Should We Stay or Should We Go” can appear heartless as the author rearranges her plot pieces into new formations with almost insolent ease, disdaining anything as cheap as an appeal for readers’ emotional engagement. It’s only gradually apparent that this sharp-elbowed satire is also a brusquely tender portrait of enduring love. In many of the chapters, Cyril and Kay end up experiencing old age together, whether it brings surprisingly successful second careers, gradual physical decline or more cataclysmic bad ends — and there are a lot of those. (Don’t even ask what happens to Cyril when he outlives Kay.) Shriver isn’t interested in reassuring us, but in the closing chapter, “The Last Last Supper,” she gives us something more satisfying than reassurance or facile sentiment: a couple honestly assessing their 57 years of marriage and affirming their commitment to each other.

Should We Stay or Should We Go

By Lionel Shriver

Harper. 288 pp. $26.99