OPEN Secrets : NOW YOU KNOW by Michael Frayn , (Viking: $21; 282 pp.)

Secrecy is sick and openness is health, we are encouraged to believe. Whistle-blower laws and a nosy press are clearly good things--the sociopolitical equivalent of free trade or keeping a bedroom window open at night.

True, there are no more shoemakers in New England, the automobile industry is rocky in Michigan, and who knows what kind of air, not to mention prowlers, is coming through the window? As for openness, could it be overrated? Secrets, after all, represent a highly organized order of intellectual energy--even if organized for slimy reasons--while disclosure, fun and of course useful, is a kind of entropy. Consider the political blahs that followed Watergate.

All this is muddy-waters, slippery-slope stuff, but it is precisely the kind of terrain that English writer Michael Frayn cultivates with such satisfactory results. Virtue is good for many things, but not for sleeping on; Frayn is an inspired insomniac and a very funny one.

In his last book, “A Landing on the Sun,” he upended two brands of respectability by unleashing an absurd and passionate affair between a proper British civil servant and an improper Oxford don. In “Now You Know” he takes a quarter-turn. Instead of mocking order with unconstraint, he mocks unconstraint with chaos. Here again, we have a civil servant subverted--a woman this time--and her subverter, a professional muckraker. Terry seduces Hilary with sex and, far more arousing, with the thrill of revealing government secrets. Only, halfway through, everything is stood on its head. Frayn’s off-balance specialty is never making an entrance from where he has exited.

Terry, who runs a tiny watchdog outfit known by its acronym, OPEN, is first heard from strutting among the government buildings in central London, as if each were a dragon he’d slain or was about to.

“We done Environment, round the back in Marsham Street. I just been up there taking a look at it, having a bit of a gloat. That story about asbestos dumping--that was ours.” He points out the Department of Trade and Industry. “Or look at that bugger, then. Sitting there with his great flat face, 200 yards from ear to ear. . . . Right. We’ll come to you in time, my son, never you fear.”

Up from the lower classes and with a bit of a criminal record, Terry is a celebrity. Members of Parliament buttonhole him, television news slots are his for the asking. Secretaries on their lunch breaks point him out in the street and civil servants on theirs glare at him. From his exuberant tones--Frayn renders voices so well that each one takes the reader on a different journey--we imagine him to be in his 30s. He is in his 60s; tall, red-faced, gray-haired, and as youthful and uncontainable as Falstaff, until he is undone in the way a prophet is undone: by a convert who believes too hard.

We see Hilary in the Home Office, the junior but promising member of a press department concerned, at the moment, with covering up the fatal police beating of a Pakistani. It is a jovial if constrained team. Hilary, who is awkward and blushes easily, who is pretty but looks intermittently like mashed potatoes, depends on this bloodless office fellowship and hates to leave work at night.

Then, through her equally awkward not-quite-lover, she meets Terry. He blasts her for representing the Enemy. She rouses herself to blast back, and both are smitten. There is a nighttime visit to OPEN’s ramshackle premises where Terry sleeps. They make comic but persuasively erotic love on the desk of Jacqui, Terry’s partner and mistress. Hilary mails him the Home Office file on the Pakistani and another on OPEN. Soon, she quits her job and, having nowhere to go, comes to work at the office.

At this point, like a top reversing itself, “Now You Know” begins to spin the other way. Like any human endeavor, OPEN is a fabric of secrets. There are Terry’s weekend trysts with Jacqui, a divorcee in her 40s who works in a state of incipient grievance because he won’t confide more of his work and sources to her. There is a pornography collection that Kevin, the office clerk, keeps in his bag. There are tensions among the rest of the staff: Liz, the zealous, mop-haired researcher; Shireen, the smilingly distant Pakistani switchboard operator, and Kent, the young black factotum who wants her job.

But these are all inside secrets. Terry takes it for granted that they have nothing to do with OPEN’s mission to uncover outside secrets. Until Hilary comes inside, that is. Terry has hidden her Home Office files on the grounds that she would go to jail if they were used. He hides from her his relationship with Jacqui, and hides from Jacqui his relationship with her. Chaos builds steadily until Hilary, seized by her newly discovered passion for revelation, manages to blow everything apart. Half the staff leaves, including Jacqui who, as it turns out--another secret--had been subsidizing the operation. Terry, pretty much blown apart himself and suddenly feeling his age, stays on, while Hilary takes charge of the remains.

“Now You Know” falls a little short of “Landing on the Sun” and Frayn’s previous “The Thing of It,” which pits a writer against a literary scholar. He is more than a satirist. In both of those novels, something wild and wonderful about the main characters reminds us that the masks of comedy and tragedy are different holes punched in the same material.

Hilary and Terry are outsized in their own way, but it is a narrower way, principally satirical. Nevertheless, they have great comic energy and distinctiveness. In one splendid scene, Hilary, having mailed her first Home Office file, wanders about London in a daze of liberation so entire that it blends into hysteria. She reaches home too late to get anything but the weather on the evening news, and even the weather report seems dazed: “The only news left is that half a dozen fried eggs are dripping incomprehensibly over a map of Britain.”

“Now You Know” is stronger in its first part than in its second. As antagonists, and in their first moments as lovers, Terry and Hilary are so distinctive that they impart tension and excitement to what goes on. Once the battle between secrecy and openness moves into the office and reverses, the tension flags. Frayn has taken fewer pains with the office staff, although Jacqui has a starchiness that is both funny and wistful. The others are barely sketches, and so the secrets they disclose and the conflicts they get into are blurred, and their momentum falters.