WITH HER NEW NOVEL, An Indecent Obsession, Colleen McCullough appears to have forsworn the model for guaranteed success she patented four years ago. With The Thorn Birds she proved she knows how to tell a good tale; as soon as her sweeping saga of love and tragedy on the Australian Outback was released, there was no doubt that it would be a hit. Whether you read it with one eye on the kids as they built sand castles on the Jersey shore or saved it to consume during coffee breaks, the epic sucked you in with its portrayal of the frailty of human passions when pitted against Destiny. The central tale of a priest's life-long affair with an indomitable, vibrant young girl who embodied the very essence of the Outback may have seemed a little far-fetched, but it worked. The book was a blockbuster.

An Indecent Obsession is a novel of modest proportions. Most of the action takes place on Ward X--for psychologically disturbed veterans of tropical warfare--in an Australian army hospital somewhere in the islands of New Guinea. Where The Thorn Birds spanned three generations, life with the inhabitants of Ward X focuses on the months after the official end of World War II. As the novel begins, McCullough appears ready to try her hand at a limited, intricate dissection of life's anomalies, rather than the universal truths she depicted four years ago. The five men on Ward X are all psychologically unbalanced, though not crazy enough to warrant being shipped off to out-and-out psycho wards at the army's expense. Nurse Honour Langtry tends this motley flock and plays the role of mother, protector and confessor to her charges. She and her patients have been patiently awaiting their discharges when the sudden arrival of Sergeant Michael E. Johnson, shortly after peace has been declared, disrupts the well-established rhythm of the lives of both patients and nurse.

The scene is thus set for what could have been an absorbing examination of the explosively tenuous relationships between the "troppo" men and Sister Langtry. But McCullough continually breaks the old Expos rule: She insists upon "telling, not showing" the complexities underlying her characters.

Sergeant Jones' arrival on Ward X leads to chaos--the reaction of "abnormals" to the presence of the "normal." Langtry, whom her patients regard as their property, falls in love with Jones. Jones, however, is "normal" and therefore a completely foreign element among the "troppos." Nurse Langtry's obvious attachment to him enrages the other patients, and secret plots and jealous machinations, leading to the murder of a patient, result.

DURING THESE GOINGS-ON, unfortunately, McCullough's superficial portraits leave the reader emotionally detached. McCullough herself unwittingly calls attention to some of the holes in her description: The hospital base, Nurse Langtry decides, has made "almost no impression at all. As if it were a stage set, without substance or real meaning of its own, simply a claustrophobic backdrop for a complicated interplay of human emotions, wills, and desires." This image applies all too well to the picture we have been given of the happenings on Ward X. In fact, the backdrop McCullough describes amounts only to the sketchy biographical information she has presented in the first few pages of the book. The "complicated interplay of human emotions" she promises exists only on those clumsy occasions when she directly states that it does. She has been unable to give us tangible, descriptive evidence for the emotions she assures us exist among the "troppos." Never do we see the characters convincingly reach to one another.

MC CULLOUGH CANNOT BE FAULTED for trying a completely new style or subject matter. But in An Indecent Obsession, she contents herself with creating the bare shell of a potentially riveting story. In describing Sergeant Jones' tendency to adopt "strays" as friends, she introduces a whole range of possible relationships between Jones and his fellow patients. But she never fully pursues the implications of that idea.

Ultimately, we conclude that in developing her outline for the nove, McCullough never fully defined her theme or her goals. We could have accepted her conception of the hospital base as a nondescript, cardboard background against which the characters of Ward X were meant to shine and attract our undivided attention. But in the course of the novel whe provides us with several extraneous alternative story lines. By the end, the novel has become only in part, the story of Honour Langtry, a 30-year-old nurse who has devoted her life to duty--"the indecent obsession." Langtry's sense of duty finally eclipses the sketchy tidbits of psychological interplay between the patients of Ward X.

When Honour decides in the final sequence to devote her life to mental health nursing, her time with the "troppos" has been over for some months. McCullough, incongruously as always, manages to squeeze in a brief treatise for social change, critiquing the mental health care system of the 1940s. Nurse Langtry suddenly and inexplicably ends the book "understanding herself at last...understanding that duty, the most indecent of all obsessions, was only another name for love."

While we rack our brains and flip back through the pages for the secret to Langtry's devotion to the patients of Ward X, we are left wondering how and why this Puritan work ethnic ever snuck onto McCullough's outline.