COMMONWEALTH

By Ann Patchett

322 pp. Harper/HarperCollins Publishers. $27.99.

In the most vivid chapter of Ann Patchett’s rich and engrossing new novel, “Commonwealth,” it is 1971 and six step­siblings ranging in age from 6 to 12 years old have been left to their own devices. Their blended family is on a car trip, staying in a motel near a lake, and the parents — the beautiful, overwhelmed mother of two of the girls and the affably selfish father of two more girls and two boys — have left a note that reads We’re sleeping late. Do not knock. Thus the children eat breakfast at a diner, then gather supplies, including soda, candy bars, a gun and a fifth of gin, and hike to the lake, where they spend several hours swimming and leaping from a high rock.

Patchett wisely underplays the drama — the chapter is a masterly example of showing rather than telling — and the increasingly shocking details speak for themselves. “The candy bars were starting to melt and the gun was hot from being out in the sun and they put them all together back in the bag.”

As a reader, I fell fully under the spell of this chapter, and I’m also pretty sure I misinterpreted it. I viewed it as an implicit rebuttal to today’s helicopter parents, an argument for letting kids enjoy their unsupervised exhilaration: “They had done everything they had ever wanted to do, they had had the most wonderful day, and no one even knew they were gone.” But as “Commonwealth” progresses, something alarming enough occurs that the case is made for being helicopter-ish after all.

Like “Bel Canto,” the best-known of Patchett’s six earlier novels, “Commonwealth” starts with an unexpected kiss at a party — in this case a kiss at the christening party for the infant Franny. Because the kiss in “Bel Canto” happens at the exact moment the party is overtaken by terrorists, it’s tempting to wonder if Patchett means to imply with “Commonwealth” that family is its own longer-term hostage situation: The embrace at the christening party between a man and woman married to other people leads soon enough to multiple divorces and remarriages. Patchett follows the two semi-connected families for the next 50 years, as the children become adults and the grown-ups become old. The nine chapters, which feel at times like short stories, proceed mostly chronologically, with regular flashbacks and occasional depictions of the same event from more than one person’s perspective. Insofar as there’s a main character, it’s Franny, who after babyhood becomes an avid reader, drops out of law school, dates a successful novelist and remains devoted to her family.