The child at issue in tomorrow night's absorbing offering from "Nova" is Genie, a teen-ager who was taken from her parents in 1970 after spending most of her life in a sort of solitary confinement, strapped to a potty chair. Her father seems to have been at best a lunatic, at worst a sadist; her mother was apparently weak and frightened. When Genie (a pseudonym) was found at the age of 13, she could not speak and behaved in some ways like an animal.

In addition to stirring the compassion of Los Angeles child psychologists, Genie presented them with an opportunity for what was known as the "forbidden experiment." The hope was that her sad condition would enable researchers to test the thesis that the ability to speak a language must be learned by puberty if it is to be learned at all. (The perfect test, to keep a normal baby isolated during the so-called critical period before puberty, is of course forbidden.)

But the question raised in "The Secret of the Wild Child" has less to do with linguistics than with scientific ethics: Is it permissible or even possible for a researcher to act also as therapist or as surrogate parent? Was there an inherent conflict between trying to help Genie adapt to her new world and using her as material for investigation?

Unlike most "Nova" reports, this is not a tale of scientific achievement. The possibility that Genie was brain damaged or retarded from birth remained unresolved, so no definite conclusion could be drawn from her faltering efforts to speak words. Hopes rose when she learned to identify colors, but she never advanced further than a sentence like "What red blue is in?" Grammar was apparently beyond her.

The program makes cool judgments about the adults who converged on Genie. At least one of the people to whom she was entrusted seems to be driven by personal aggrandizement, and others do not seem to have been fastidious researchers. Even those with the most honorable intentions worked in sometimes conflicting directions.

Finally, after paying for years of unproductive studies, the National Institute of Mental Health cut off financing, and the researchers moved to other projects. Genie was relegated to foster homes, and her case wound up in the courts when her mother, of all people, sued the professionals involved for letting their testing take precedence over the girl's welfare.

The appealing qualities that all the researchers found in Genie come through in the pictures here, taken from hundreds of hours of videotape of the much-studied teen-ager. We root for her to succeed. But you will learn more about those who tried to help her or study her or exploit her than about Genie herself. Their secret is that they are human, too. NOVA The Secret of the Wild Child PBS, tomorrow at 8 P.M. (Channel 13 in New York) Written, produced and directed by Linda Garmon. Joseph McMaster, associate producer; Don Lenzer and Boyd Estus, cinematographers; Elmer Bernstein, music; Alexandra Anthony, editor. Produced by the WGBH/ Boston Science Unit in association with BBC, NDR International Hamburg and Nederlandse Omroepprogramma Stichting; Paula S. Apsell, executive producer; William Grant, executive editor. Narrated by Stacy Keach.

Photo: The teen-ager who is known as Genie has spent most of her life in solitary confinement. (Dr. Jay Shurley/Nova)