The mystery writer Robert Harris likes to place his murder stories in specific historical situations. His first, “Fatherland,” was set in Berlin, the capital of the Third Reich, in a future in which Hitler had won the war. His second, “Enigma,” concerned strange goings-on among World War II Bletchley Park code breakers. In “Pompeii” and “Imperium” he has opened up a second front, venturing into the Ancient World.

Before he became a writer of commercial fiction, Mr. Harris was one of Britain’s most astute newspaper political commentators, turning his eloquent and often acidic pen on the governments of Margaret Thatcher and John Major. He wrote a generous biography of the Labour leader Neil Kinnock and a rather less kind one of Ms. Thatcher’s devoted press secretary, Bernard Ingham. In the course of his journalism Mr. Harris came to befriend Tony and Cherie Blair and, though he did not live in Islington, the inner London suburb that was the home and heart of New Labour, he became an intimate in the broad circle of bright meritocrats surrounding the young Labour leader and his wife. Which is why his latest yarn, “The Ghost” (Simon & Schuster, 335 pages, $26), is such an unsatisfying tease.

“The Ghost” tells of a ghostwriter hired to write the autobiography of Adam Lang, a charismatic and handsome young British former prime minister who suffers from a hysterical and overbearing wife. While trying to finish the ghosted memoirs in a borrowed villa on Martha’s Vineyard — a location picked, as so much in this book, to pander to the American market — Lang is cited as a war criminal by the International Court of Justice for his part in the war on terror.

The parallels between Mr. Blair and Lang are so plain that Harris omits one essential element of the roman à clef: allowing readers to make their own deductions and come to their own conclusions about who is meant to be whom. In “The Ghost,” everything is clearly labeled, lest the dumb reader miss the obvious connection.

Mr. Harris gets Mr. Blair right. “Everybody voted for him. He wasn’t a politician; he was a craze,” could only be about the Blair phenomenon, part one-hit pop star — of a particularly bland English sort that doesn’t travel — part Rubik’s cube.

The description is telling. Mr. Harris came to tire of Mr. Blair’s silver tongue and slick presentations, and became disillusioned with his failure to enact domestic reforms. Asked what single word would best describe Mr. Blair after 10 years in the job, Mr. Harris responded without a blink, “Conservative.”

But as the book progresses Mr. Harris finds Mr. Blair so lacking in color — or perhaps he is unable to find adequate words to fit Mr. Blair’s constantly shifting personality — that he has to reach out to the life of another prime minister to construct a rounded character.

“It’s losing power — that’s the real trouble,” Lang’s wife says. “Losing power, and now having to sit down and relive everything, year by year. While all the time the press are going on and on about what he did and didn’t do. He can’t get free of the past, you see. He can’t move on.” From the moment she was deposed by her peers in 1990, Ms. Thatcher has pined for the power she wielded so effectively — and so exclusively — for eleven years.

Mr. Blair, meanwhile, seems perfectly happy without power. He has acquired at least the trappings of influence as the Quartet on the Middle East’s envoy and it may be that first class hotels and travel, an armed guard, and a police escort will be enough to satisfy him.

If you take away the capacity for self knowledge, the following, from Lang, is pure Blair.

“All my political life, whenever my opponents have been really stuck for something to hit me with, they’ve always said I was a f— actor.” He sprang up and started striding up and down. “Oh, Adam Lang,” he drawled, performing a pitch-perfect caricature of an upper-class Englishman, “have you noticed the way he changes his voice to suit whatever company he’s in?” “Aye” – and now he was a gruff Scotsman – “you can’t believe anything the wee bastard says. The man’s a performer, just piss and wind in a suit!”

Mr. Harris gives Mrs. Blair an even less attractive persona. Take this: “She was wearing a man’s thick, shapeless white sweater, so long in the sleeves that only her chewed fingernails were visible. … Her short dark hair stuck up in Medusa-like spikes.”

It would be interesting to know what Mrs. Blair has done to so upset Mr. Harris. Then again, perhaps it wouldn’t. There is no suggestion that Harris is settling old scores here; he appears to be borrowing from what he knows to help build characters on the cheap. There is no sense that he genuinely intended this crime thriller to be a disguised history of the Blair marriage. Mr. Harris’s parade of ready-made historical backdrops, it seems, are merely ways to avoid having to imagine a world of his own.

Which is why, if you want to solve a whodunit on the beach or on a plane, you would be better off with someone who can write really well, such as P.D. James. And if you are more interested in the dreary domestic doings of the Blairs, you would do better to read the “Diaries” of Mr. Blair’s hectoring press man, Alistair Campbell. When it comes to describing the private life of a prime minister, Mr. Campbell writes much better fiction.

Mr. Wapshott’s biography of Ronald Reagan and Margaret Thatcher will be published by Sentinel, an imprint of Penguin USA, on November 8.