Barbara Kingsolver is a prod to the nation's conscience.

Her novel "The Poisonwood Bible" laid bare misguided missionary zeal. "Prodigal Summer" established our interconnectedness with the larger plant and animal world. As founder of the Bellwether Prize for Fiction, she's encouraging a new crop of social novelists.

"The Lacuna" fits into this tradition. It explores the social and historical context of America's reactionary fear of those who presume to question our system of government. The author stirs the real with the imagined to produce a breathtakingly ambitious book, bold and rich. If her dramatized history lesson feels at times forced, it also feels important.

Where the book sags under the weight of so many good intentions, two vital female characters prop it up: the saucy Mexican artist Frida Kahlo and the fictional archivist-stenographer Violet Brown.

"The Lacuna" covers the years 1929 to 1951 in the life of blue-eyed Harrison Shepherd, born of a Mexican mother and an American father. A portmanteau of two cultures, Shepherd is the novel's shy, repressed protagonist. Torn between the cultural heritage of parents who wouldn't earn the Good Housekeeping seal of approval, Shepherd is that odd breed, an "orphan boy" with two living parents.

Harrison is taken to an island off the coast of Mexico at age 12 by his mother, who has become attached to a Mexican attache. Ignored by a mother who is foulmouthed, colloquial and colossally selfish, he is mostly paid attention to by members of the hacienda's staff. He finds comfort in words scribbled in notebooks and in swimming through openings in the coastal cliffside called lacunas.

The lacuna is the novel's controlling metaphor. It is a gap or missing part, and the word keeps turning up, a seaside penny, to suggest our incomplete understanding of others' stories.

Before long, Harrison's mother ships him to America to his father, a government "bean counter." Enrolled in a military academy, the teenager witnesses the Bonus Army riots of homeless veterans and experiences his own internal riot when he has an affair with a scholarship student.

This "irregular conduct" gets him booted from military school and sends him back to Mexico, where he ends up a plaster-mixer for the muralist Diego Rivera.

Rivera's wife, artist Frida Kahlo, becomes Harrison's ally and confidante. She gives him the nickname Insolito after she learns of his military school "conducta insolita," or irregular conduct.

The Kahlo-Rivera household simmers, a rich and colorful brew of talent, politics and infidelities. At night, Harrison records events in his journals, earning him a job as Rivera's secretary.

A pastiche of letters, notebook and diary entries, invented and real newspaper and magazine articles, "The Lacuna" dares to question America's historical myopia and a national history full of gaps.

Kingsolver's seventh work of fiction is hopeful, political and artistic. "The Lacuna" fills a lacuna with powerfully imagined social history.