HEREWITH Kramer vs. Kramer from the distaff side: a novel about a young mother who has custody of her 4-year-old daughter, then loses it to her former husband after a bitter court proceeding. It's a first novel, with all the predictable characteristics and shortcomings of apprentice work: a self-absorbed first-person narrator, labored expository passages, prose that reaches for an excess of lyricism, and a plethora of extraneous detail. Yet though its artlessness can be distracting and irritating -- especially in the first 150 pages, before Sue Miller gets command of her story -- The Good Mother is redeemed by its author's not inconsiderable strengths: a willingness to cope with serious, complex issues, and a sincerity that is genuinely appealing.

The mother of the title is Anna Dunlap, whose loveless marriage to Brian, an ambitious lawyer, is ending by mutual consent. She is a bright, rather excessively self-conscious woman who yearns to escape from her past, "where I was surrounded by love, by protestations of love; but love conditional on so much: on being good, whatever that meant; on doing well; on making the family proud." She longs for "independence," for "being my own person," and for a life without constraints in which the passions she has suppressed since girlhood can at last be set free.

But she also wants a safe, ordinary, secure life with Molly, her daughter, with whom she sets up housekeeping in an apartment in an unprepossessing part of Cambridge. Brian has agreed to satisfactory child-support payments; her hope is to support herself by giving piano lessons and, as soon becomes necessary, taking a regular job. Her life is quiet, bounded by her undemanding work and her devoted attentions to Molly; a friend or two keeps her in touch with the outside world, but she has no romantic life and no apparent interest in one, even though she admits that "I didn't want to be as solitary as I was, as I'd grown used to being."

All that changes with the unexpected arrival in her life of Leo Cutter, a dynamic artist who is as determinedly bohemian as Brian was inflexibly conventional: "From the start, we fought and then made love, both with a passionate intensity that I had thought as lost to me as the possibility of making great music. I felt I'd been traveling all my life to meet him, to be released by him. I felt I was . . . another version of myself, another model for being." With him she becomes at last what she had so badly wanted to be, "a passionate person," and she fancies that with him and Molly she can live "life without limits."

BUT LIFE does not work that way; life has limits. She comes to understand this when Molly, on a visit to her father and new stepmother in Washington, blurts out something that leads Brian to believe she has been approached sexually by Leo. Without giving Anna an opportunity to deny or explain, he goes to a lawyer and files a demand for custody. When Anna confronts Leo with the charge he does not deny it, but offers a reasonable explanation, the brunt of which is that when encountered in the shower by a curious Molly, he did what he imagined her newly liberated mother would have wanted him to do. It was all, he and Anna uncomfortably agree, a misunderstanding.

But the courts, not surprisingly, feel otherwise. There is no real suspense about the outcome of the case -- whether Miller intends this to be so is unclear -- because the incident between Leo and Molly reflects, in the eyes of society, behavior too unconventional to be acceptable. That Anna has been a good mother is not seriously disputed by anyone, but so far as the court is concerned her passionate affair has blinded her to parental responsibilities and accustomed her to behavior that could be damaging to the child. Though a neutral psychiatrist strongly recommends that Molly remain with her mother -- not merely because Anna has been responsible and loving, but also so as to maintain continuity -- the judge decides to the contrary.

There are elements of courtroom and domestic melodrama in this, just as there were in the considerably more artful Kramer vs. Kramer, but they are of less consequence than the serious issues Miller raises. The central one is that there is no such thing as "life without limits" and that one of an adult's most important obligations is to discover what those limits are: to learn, that is, the rules by which one's life must be lived. Closely related to this is the clash between the emotions Anna feels: the quiet, steady, abiding love for Molly, and the wild, tempestuous, consuming passion Leo arouses. Caught up in the latter, she allows the former to recede into the background; had she been more vigilant, less swept away, the offending incident might never have taken place.

While these matters occupy the forefront, in the background is Anna's relationship with her maternal grandparents, sternly old-fashioned people whom she loves but who represent precisely "the world I'd emerged from, the world I thought I was shaking off." The most effective scene in the novel occurs when Anna, confronted with her lawyer's $2,500 retainer for the custody case, goes to her grandfather for money; not merely does the encounter place her in direct conflict with the world of her past, it also shows the unexpected divisions and rivalries within that world itself. From her grandparents as from the court, she learns that you can't have it all.

That's one of several sound, refreshingly adult conclusions that Miller reaches in The Good Mother, so it's all the more a pity that the novel is considerably less successful as a work of fiction. If ever an author has had a show-and-tell problem it is Miller, who belabors her thematic material to a fare-thee-well; though she has woven this material quite successfully into her story, she insists on examining it didactically as well, apparently not trusting the reader to discover it on his own. She also takes far too long to get her tale into place; not until she has given us prolonged, repetitious background material on Anna does she at last move into the heart of the book, the custody case, and only then does The Good Mother come alive. She gets high marks for seriousness and sincerity, but -- even though The Good Mother can be recommended as intelligent and provocative -- low ones as a novelist.