"Promises in the Dark" -- opening today at the Inner Circle -- is a woefully unedifying account of the friendship that evolves between a doctor, Marsha Mason, and her doomed patient, Kathleen Beller, an exuberant high-school girl stricken with cancer.

Under the best of circumstances this story would promise a bleak evening's entertainment. But after screenwriter Loring Mandel and director Jerome Hellman finish treating it with more earnest superficiality, the movie itself is a goner.

After interminable motoring under the credits, Mason arrives in Hartford, Conn., to become the junior partner in a practice established by a kindly old family doctor played by Donald Moffat. While her mentor is away, Mason inherits the emergency case of the young Miss Beller, whose ankle injury uncovers an advanced bone cancer, requiring immediate amputation of her leg.

The situation would have more dramatic leverage if the girl and her doctor had been previously acquainted. Making the doctor unfamiliar with both her surroundings and her patient seems to retard the exposition.

In addition, a number of obligatory scenes are missing -- including one in which our supposedly straightforward physician would take the trouble to tell the girl what was wrong before sending her in for decisive exploratory surgery. It seems a little thoughtless of someone when Beller wakes up with a missing leg and Mason belatedly breaks the bad news to her.

This oversight looms particularly large in the context Mandel is determined to create: The doctor is meant to defend the dignity of a dying patient against the hand-wringing, self-centered ignorance of the girl's parents, a creepy misalliance of Susan Clark and Ned Beatty.

Mandel's lack of generosity to these people may betray his allegiance to the grandstanding tradition of topical television drama. The fussy, inept parents provide a weak moral alternative to the resolute physician, who may be jeopardizing her career by performing the merciful gesture they can't bear to perform: pulling the plug of the life-support systems when all reasonable hope of recovery seems lost.

One ever feels a human or dramatic need for this loaded contrast, and it shifts attention away from the ordeal of the patient herself. Mandel's evident need to characterize the parents as pathetic figures makes one doubt the soundess of his endorsement of the good doctor. It's always easier to know the right way to act when faced with a terminal illness in someone else's family. Again, this weakness might have been avoided if the doctor and family had been close to begin with.

Kathleen Beller is such an attractive young actress that the idea of her wasting away from a debilitating illness has a special quality of alarm. Her blooming, dark-eyed beauty and youthfulness are ideal for emphasizing the injustice of it all. Unfortuately, her character's misfortue begins to look unbearable in the wrong way -- not so much affecting as lugbrious ad unsightly.

Beller and Paul Clemens play effectively as high-school sweethearts whose affections fail to survive her pain. Mason is given a parallel affair of theoretical but unconvincing therapeutic value with Michael Brandon as an available radiologist.

In th early scenes Hellman shows a flair for directig setting that teem with activity -- a high school playing field and the hospital -- but the grows progressively listless when compelled to concentrate on intimate encounters. Of course, the sort of intimate encounters Mandel invents might immobilize the most vigorous of directors. By the time it expires, "Promises in the Dark" suggests a moviegoing form of euthanasia.