Karen Valentine has returned, this time as a human being. The sprite who once perked, twinkled and cuted her way through programs like "Room 222" shows admirable if still slightly ingenuous backbone as "Muggable Mary, Street Cop," the CBS movie at 9 tonight on Channel 9.

Either all perked out, or resolved to prove herself a serious actress, Valentine brings warmth, strength and moxie to the role of divorce' Mary Glatzle, who joins the New York Police Department because it has "a great medical plan" and because she has an 8-year-old son who was born hydrocephalic, has had seven operations already and may need more.

Her six months of training are capsulized into about 14 minutes in the screenplay by Sandor Stern, who also directed; unfortunately, a fakey rivalry is contrived between Mary and the judo instructor, Frank Reineke (John Corcoran), and any viewer knows it will be resolved by having Mary knock the wind out of him. Much about the film--based on Glatzle's real-life experiences--is too pat and tidy, but Valentine glorifies it into something approaching honesty.

Her face has aged enough to give it character. She still has pudgy little cheeks, but now they're, well, expressive pudgy little cheeks. Somewhat wittily, the producers cast a pudgy-cheeked youngster, the very natural Michael Pearlman, as her son, and this relationship is dramatized without forced sentimentality. The rest of the film consists mainly of vignettes involving Mary in the hard world of crime. After her first encounter with a homicide victim, she cries, but she returns to work the next morning, and later sets a record for arrests playing decoy for rapists and muggers.

The film goes very, very well until the second half, when it suddenly goes very, very wrong. Joe Bell, a black cop played by Robert Christian, is killed on duty while Mary and partner Dan Waters (John Getz, who has a nice, open grin) are waiting for him in a squad car. Her reaction to this incident is limited entirely to selfish shock that she might have been the one killed. The late Mr. Christian is not given a single thought. He had been an ingratiating character, but he is never spoken of again. A movie that had seemed awfully positive turns oddly ugly.

The picture never recovers from this beastly miscalculation, but Valentine's performance is unblemished by the writer's insensitivity. "Muggable Mary" may have series potential, but that would probably mean a cross between "Alice" and "Private Benjamin." For now, it is encouraging to see Karen Valentine with her new-found rough edges showing. Rough edges can be beautiful and, in this case, are.