There's a distinct fascination in reading about one of the more bizarre forms of time-travel available to us in this century -- that extraordinary process through which an apparently ordinary "girl" is transformed into a Roman Catholic nun. The tortured journey that Karen Armstrong recounts in "Through the Narrow Gate" was bitterly resisted by her rollicking, life-loving English parents -- but not by her gintippling, sexually promiscuous grandmother, who revealed she once thought she'd had a vacation herself.

The most startling aspect of her immersion into that austerely self-crucifying (but unidentified) religious order was its time-frame: she enters as a naive, damply idealistic convent-school graduate in 1962, and was released from her vows seven years later, when she was reading literature as an Oxford under graduate in 1969. This was the prime Beatles era, the decade of the Pill, Vietnam, protest marches youth revolt, rising feminist rage -- when the "winds of change" were also blasting the Catholic Church. Long after the fact, Armstrong acknowledged the irony that she then "had no idea that I would be one of the very last postulants to be trained along the old lines of severe victorian discipline."

As a lumpen, bookish, introverted teen-ager determinedly "beginning a huge spiritual adventure," she was still enough a product of the '60s to react with incredulity to her discovery of the knee-length bloomers and the green carbolic soap that would also serve as her shampoo during the once-a-week bath: "No more Tampax, no more deodorants, and -- horror of horrors -- only two pairs of knickers a week!" Yet, as Sister Martha, her only news of the outside world for the next three years came when she was told to pray for the world, which was about to be blown up in the Cuban missile crisis. (After three terrified weeks she dared to ask if there was any further news, and learned "that cleared up weeks ago.")

There is the usual quota here of twisted and sadistic religious superiors, vengefully assigning tasks like practicing for weeks on a sewing machine with a broken needle, or scrubbing a basement staircase with a worn pink nylon nailbrush. There is also the usual litany of peculiar religious rationale: "Even if Mother was mistaken, my job as an obedient religious was to accept her decision blindly, because that would be what God wanted."

But for Sister Martha there were particular trials. Severely allergic to certain foods but bound by obedience to eat them, she vomited up her first convent supper. Later, at Oxford, when she was sick nearly every night," she was finally taken to a doctor, who diagnosed incipient anorexia nervosa. This was instantly translated into nun-language: "'You mean,' Mother Constantia said after a puzzled pause, it's just her nerves.'"

"Nerves" also explained her periodic blackouts in the chapel, followed by paralyzing headaches: "How dare you, Sister! . . . You do not disturb your sisters' prayer by falling about on the ground in that disgusting display of hysteria."

Among the various psychic horrors she recounts, a few were specifically sexual. Her experience of "sex" was-limited to a necking scene she once witnessed between her cousin and her friend Suzie -- and, much later, the grapplings of a 60-year-old priest: "How right I'd been to reject all such unpleasantness." But the first time she was ordered as a novice to practice "corporal mortification" with a knotted whip known as "the discipline," she experienced a strange arousal: "As I went on I no longer felt the pain. Just this dark and restless excitement . . . and then there was a huge sense of release."

Troubled, she confessed to her superior that she found the experience "exciting." She was ordered to "persevere. Beat yourself harder. Make it unpleasant and painful . . . You are beginning to die. It's a slow process; you'll be at it all your life. But you're beginning.

Only years later, when her novel-reading at Oxford had stirred "temptations that . . . revealed my basic ignorance," did she find the courage to defy the rule to "take the discipline," telling her scandalized superior, "To me it seems that flagellation is a means -- a perverted means -- of sexual satisfaction."

When she was finally released from her floor-scrubbing to "cram" English literature, she "went dizzy with joy: Jane Austen, Keats, Wordsworth, George Eliot. I felt like a starving person about to sample a banquet." Yet, still the good nun, she "tried to still my joy. I shouldn't take pleasure in this . . . [I] fought my way hopelessly toward indifference."

At Oxford, bewilderingly adrift in a world of miniskirts, easy sexuality and intellectual challenge, she gradually honed her rusted mental acuity. But the exhausting, schizophrenic mix of her exhilarating tutorials and her spiritually arid convent life led inexorably to a total breakdown.

On the first night of a summer retreat, with a "strange, keening scream that sounded like an animal caught in a trap," she collapsed into the arms of the nuns who were trying to slap her into silence: "Then something snapping. The sound stopped. Two selves pulled apart. Blackness."

Armstrong tells her short, brutal tale with such deft and dispassionate objectivity that we comprehend the missing core of her convent experience -- the embrace of a loving, supportive community of women -- only in the last, best pages, after she made that necessary but still resisted the decision to leave.

"Dispensed" only that morning from her vows, stripped of her gold profession ring and oversized rosary, she checked into an Oxford student dorm to begin the next term. Virtually frozen with fear, still wearing her swaddling black habit, she forced herself to seek out two Catholic classmates.

As Rose and Bridget, with tactful astonishment, pulled the news of her leaving from her, they responded instinctively with the warmth and compassion so long absent from her life. In a re-robing ceremony eerily reminsicent of her earlier march down the chapel aisle to become a nun, they launched a closing hour foray of Marks and Sparks department store. In that brief, dizzying transformation of Sister Martha back into Karen Armstrong, this book finally comes alive -- and, mercifully, she ends it there.

In a brief afterword written 11 years later she admits, "I had no idea how truly terrible the period of adjustment was going to be." When she confided to her only friend from convent days, an Oxford classmate now also an ex-nun, that she had written this book, the response was, "How amazing." Then, "It's time . . . How brave of you."

True enough, this is a brave document by a gifted but very damaged woman, who still considers herself "a better nun now than I ever was in the Cloister." For her the writing of it seems to have been an act of personal exorcism. But for us its main value is as a startling footnote to the annals of psychic barbarism in these "modern times."