But there’s nothing fantastical about Fowler’s new novel with its drawing-room title, “We Are All Completely Beside Ourselves.” In fact, the plot is inspired by several real experiments, including the work of Winthrop and Luella Kellogg, scientists at Indiana University who raised their baby son alongside a chimp for almost a year in the early 1930s.

Fowler places her story in the 1970s and extends the experiment to five years. Dr. and Mrs. Cooke live in a farmhouse with a gaggle of graduate students in Bloomington, Ind. They have a son named Lowell and two new daughters, Rosemary and Fern. Rosemary never stops talking; Fern never starts. But their parents have “promised to love them both exactly the same.” So far, so normal.

In a witty, conversational voice, Rosemary reluctantly parcels out the details of her “chimped-up household.” She doesn’t mention her sister’s body hair issue until page 77. “I spent the first eighteen years of my life defined by this one fact,” she says. “It’s never going to be the first thing I share. . . . In my defense, I had my reasons,” she adds. “I tell you Fern is a chimp and, already, you aren’t thinking of her as my sister. You’re thinking instead that we loved her as if she were some kind of pet.” She’s right, of course. Fern’s identity is the 800-pound gorilla in the room. The mechanics of this weird family arrangement are irresistible: How did the Cookes care for these two toddlers, feed them, dress them, keep them from hurting each other? “What was the goal of the Fern/Rosemary Rosemary/Fern study before it came to its premature and calamitous end?”

As an adult looking back on her famous childhood, Rosemary is curious about those questions, too. But the answers are elusive because once Fern left the family, no one mentioned her again, and it’s not at all clear what precipitated her departure. All Rosemary can do now — many years later — is try to excavate memories of their time together and catch lingering impressions of her sister still persisting in her own personality.

All this sounds like rich material for a novel, but there’s more. “We Are All Completely Beside Ourselves” isn’t just about an unusual childhood experiment; it’s about a lifetime spent in the shadow of grief. Clearly, something traumatic happened when Rosemary was 5, something that turned her from a loquacious little girl into a quiet young woman. But unearthing the details of that event means digging in a mental landscape strewn with psychological land mines. Others can’t or won’t tell her the truth. Her own memories are confused and clouded. She’s grown wise and skeptical about the slippery nature of family history. “Language does this to our memories,” she says, “simplifies, solidifies, codifies, mummifies. An oft-told story is like a photograph in a family album; eventually, it replaces the moment it was meant to capture.”

Although the story moves erratically over almost 40 years, it focuses on a few chaotic days in 1996 when Rosemary was a fifth-year student at the University of California at Davis. An unlikely friendship with an unstable fellow student triggers a series of confusing feelings. “This, finally, was the moment the hypnotist snapped her fingers,” Rose says ruefully. Curious but wary, and with a wry reference to the damage done by Sigmund Freud, she begins reconstructing what happened to her and her family, handling old memories worn “thin as Roman coins.” Refreshingly, she has the humility to admit that she can’t tell whether she’s making some of this up: “I was completely buried in the unremembered, much disputed, fantasyland of the past.”

Plot is not the novel’s strongest suit. The wackiness that stumbles into the final chapters feels incongruous with the book’s poignancy and its serious themes. But Rosemary’s voice and her efforts to understand — and forgive — herself are moving. Fowler has such a sprightly tone, an endearing way of sloughing off profound observations that will illuminate your own past even if you have no chimps swinging in your immediate family tree.