A Sex Comedy of Lags and She-Lags : THE PLAYMAKER <i> by Thomas Keneally (Simon & Schuster Inc.: $18.95; 327 pp.)</i>

At the narrative climax of Thomas Keneally’s “The Playmaker,” our hero Ralph Clark gives panting chase to a ghostly figure in a white suit. It is night in Australia, and the moon is crazy blue. Ralph is a lieutenant of H.M.’s Royal Marines, an honest chap with a spendthrift wife back in England, eight moons away.

In the dark, Ralph assumes the fleeing figure is Dennis Considen, the Irish dentist. Wrong assumption: It is Mary Brenham, a “she-lag” (convicted felon deported to Australia). Transported at age 14, for seven years, for stealing clothes valued at 39 shillings, beautiful, semi-helpless, joyously lustful Mary has been cast by playmaker Ralph in the part of Sylvia for George Farquhar’s 18th-Century comedy, “The Recruiting Officer.”

On the night of the climactic chase, Mary rehearses her lines in a cedar grove, donning her player’s white suit as she dons her Sylvia stage mask. Costumed, stepping on stage, the comely she-lag escapes the dirty, filthy, mud-stained muck of the Lagtown she lives in.

The year is 1789. The place is Sydney Cove, Australia, Britain’s version of Devil’s Island. Royal Marines live in rude wood soldier’s huts. Convicts live in mud huts across the stream in Lagtown. The event of the book is Farquhar’s play, a comedy to celebrate the king’s birthday on June 4. The theme is Art versus Reality.

In Part 1, clergyman Dick Johnson warns Ralph that the play is seductive. The language, he cautions, will inflame the felons. Nonsense, says Ralph. This is a comedy. Use your imagination.

And yet, here he is in Part 5, an officer inflamed, tearing the white calico playsuit off the lush body of Mary Brenham. Mary understands; she yields. She is a convict. Ralph is an officer, a playmaker. He will build a hut for her in his garden. Art wins out. “The play and Ralph and Mary,” Keneally writes, “were of one mind.”

On the night of the play, the night of the King’s birthday, a minute away from the Act II curtain, the Night Watch appears at the rude theater with Black Caesar, rapist and mad Madagascan, wearing 28 pounds of leg irons. Caesar, a huge mythic lad, sucks up audience attention, exploding two months of sweaty convict rehearsal.

Mary, fearful that as a victim she will have to testify against Caesar, dashes away. Again, Ralph gives chase. This is his she-lag, and his play is coming apart at the seams. “I’ll not go to court,” Mary wails, “and show my arse.” Put on your mask, Ralph argues. You are Sylvia.

Does Ralph cajole her back on stage? Does Black Caesar steal the show? Will Reality triumph in Sydney Cove? Or will Art?

“The Playmaker” is Keneally’s 16th novel. He’s an award-winning pro, weighing in with careful research and heavy themes. But to make this soft-core history stuff fly, you need every rivet in place.

I have two beefs with “The Playmaker.” First, the point of view often wobbles out of focus. Second, erratic flashbacks distort the chain of events.

The book opens with this line: “First Ralph heard again, how Harry had, one evening in the settlement’s first days--discovered Duckling’s absence from her tent across the stream.”

The information is second-hand, Harry retelling to Ralph, with Ralph as a screen between the reader and the scene. The second verb is past perfect, plunging us through a trap door into back story instead of propelling us into the tale. This first sentence is a stylistic hint of what lies ahead. You have to read 150 pages, working at it, before the book gains momentum.

On the upside, the language feels 18th Century. The book has that hurly-burly structural quality of “Tom Jones,” with a layer of naughty wit a la “Tristram Shandy.” The hanging scenes are nicely done. So is the capture of the aborigine , Arabanoo, who develops a taste for brandy, which he christens “The King.” But I kept wanting Keneally to write from the point of view of a convict. (What was it like to be a “lag,” transported for life to cozy Sydney Cove?) I wanted Ralph, honest man of letters, out of the way.

In Part 5, just before the arrival of Caesar in leg irons, playmaker Ralph lets go: “He had a sense that his players had somehow become their own actors, independent of him like grown children . . . that they had entered a pact with the audience which was rightly none of his business, and that only the approval of the crowd could justify them. . . .”

Message to Keneally: Behold thy playmaker, Ralph. Exit the stage. Let your actors act.

“The Playmaker” contains wonderful scenes. It will make a hummer of a movie.