Leave of Absence<i> by Ralph McInerny (Atheneum: $14.95; 256 pp.) </i>

A nun starts an affair with her best friend’s husband; the best friend, in turn, contemplates running off to become a nun. This is about as promising a start to a wicked little Catholic farce as has come along since Sister Mary Ignatius explained it all to us, but “Leave of Absence” isn’t played for laughs. It’s meant to be serious stuff--post-Vatican II contemporary Catholic identity crises--and that’s already giving it far greater due than this fluffball of a book deserves. It’s serious, all right; seriously bad.

This hodgepodge of flashbacks, flash-forwards and flash God-knows-where opens in Rome, where Vera, the 37-year-old protagonist, has gone to lick her wounds after husband Edward the dentist leaves her for best friend Andrea the nun. Vera has taken a leave of absence from her law firm, ostensibly to check in on her troubled niece, who has gotten in over her head with a, yes, young seminarian. Clerical coupling is what you might call a theme of this book; leave-taking is another. Everyone, including the minor characters, takes a break from his or her chosen “vocation”: Vera and Edward from their marriage vows, Andrea from her religious vows, Andrea’s ex-nun friend Alma from her religious and her marriage vows (to an ex-priest).

It may all sound steamy enough (some steam would have helped), but McInerny, editor and publisher of the journal “Catholicism in Crisis” and author of the popular “Father Dowling” mystery series, is trying to make a real point, about vocations. Not a bad concept; the idea of a vocation--an inner sense of being called, by God, to your role in life--is fertile enough ground in these times when so many seem to have trouble committing to anything. The trouble is, McInerny has peopled his book with characters who have only the most superficial notion themselves of what a vocation is.

Take Andrea--her attraction to the religious life is largely to the romance of the veil and the shelter of the convent. When her order, in post-Vatican II spirit, abandons the habit and allows the nuns to live on their own (a move she originally opposes, then espouses), it isn’t too long before Andrea is wearing makeup and bikinis and sleeping with Edward. Likewise, it’s the sight of the effigy of a nun in full habit, encased in an altar, that inspires the divorced Vera to join the religious life, “the life whose symbol was the religious habit she wore.” The fact that these two women’s sense of vocation is so heavily tied to costume speaks exactly to the reason many religious orders decided to abandon them in the first place.

With their understanding of vocation cloth-deep, it’s no wonder these characters seem to feel they can slip theirs on and off as easily as they would a pair of socks. But McInerny, whose bitterness over many Vatican II changes is apparent throughout, gets them in the end. As they sort themselves out, though none happily, the moral of the story seems to be that your vocation isn’t just your calling, it’s your prison.

That’s not what one would call a very positive Catholic message--he’s not editor of “Catholicism in Crisis” for nothing--but by this point, we’re way past caring anyway. The novel’s real problem is that McInerny has given us nothing and nobody worth our emotion. There isn’t a single relationship that’s drawn convincingly, especially that of the two “best friends,” who seem so merely cordial, it’s hard to believe they’ve ever even met. Instead, we’re treated to a lot of high-minded, pointless name-dropping (Vera’s a Thomist, we’re told, while Andrea is a devotee of Duns Scotus; what this means we have no idea), and a lot of, you know, contemporary Catholic dialogue:

Vera: “What’s the point of being a nun if you dress like everyone else, live like everyone else and make your living like everyone else?”

Andrea: “We are the new nuns, Vera.”

McInerny may have felt he was providing some service with this story, showing how nuns and priests are people, too. Unfortunately, all he’s shown is that nuns and priests, in the hands of a bad writer, can be just as shallow as any lay character.