A massive layer of irony and some strong adult characters save "Happy Face Murders" from being just another killer-of-the-week movie, although there is enough revolting blood, guts and profanity to satisfy adolescent minds. It airs on Showtime tomorrow night.

Ostensibly based on a real crime, screenwriter John Pielmeier (formerly the playwright who wrote "Agnes of God") and director Brian Trenchard-Smith immediately set the tone with music from the cutesy teen musical "Bye Bye Birdie," the song "Put On a Happy Face." The tune does double irony duty--a wry counterpoint to the serial killing of truck stop prostitutes that constitutes the crimes, and an in-joke reference to one of co-star Ann-Margret's early movies. Are we clever, or what?

Ann-Margret, who has almost made the transition from sex kitten to Grande Dame Artiste, plays the weirdest character in the show--a 58-year-old grandmother who is locked in an enabling relationship with an abusive nut case 20 years her junior (Nicholas Campbell as Rusty). By the way, we know exactly how old everyone is because they are all introduced with a subtitle giving their name and age. Anyway, Ann-Margret's Lorraine Petrovich, a nurse's aide in the nearby mental hospital, wears a bad old-lady wig and frumpy clothes, and watches way too much television. And she can't figure out how to get rid of her bad boyfriend.

The plot revolves around the death of a mentally retarded woman who has been brutally murdered by someone she picked up in a bar. Exactly whodunit is the engine of the plot, made more complicated by a series of false confessions and accusations.

The real engine of the movie, however, is Marg Helgenberger (you can tell she is from a different generation than Ann-Margret, given that she didn't change her name). She plays Jen Powell, a tough, funny, smart police detective who also happens to be impossibly beautiful. This is the kind of character people want when they complain about the dearth of positive female roles on television--she's not passive, a victim, stupid, lonely, man-hungry or warped. She also defies her bosses and solves the crime after they'd settled for convicting the wrong people. What a gal!

Among other quirks, Powell takes in stray dogs for the ASPCA, which she names after politicians so that when she has to give them up she won't feel so bad. Thus she's seen walking Janet Reno and Jesse Helms in tandem, and Kenneth Starr eats dog food. She names one pooch Bob Packwood so she can tell her ex-husband, "I finally found someone to replace you, Al."

Her sidekick is a young graduate student (Henry Thomas, very far from "E.T."). This device provides ample opportunity for references to the wisdom, etc., of older women--somewhat belied by the sterling example of Lorraine and Rusty.

Like I said, the people who made this film are entranced with irony, the intellectual crutch of our age. Television detective shows ("Perry Mason," "Matlock") are always playing on Lorraine's TV, and people says things like "You can't believe everything on television." There's a section where the color flips around, with some characters in black-and-white and some in neon. Very arty.

But the tone works for the story, which is so bizarre that the self-referential bemusement is effective (to reveal much of the ups and downs of the plot would probably spoil it). The profanity was startling for me, a nonsubscriber to Showtime, but is really no more than what you'd hear in the average office, especially one where people are dealing with awful homicides. Not so the bare breasts, however--at least not in any workplace I've seen.

CAPTION: Ann-Margret plays a frumpy grandmother who watches way too much TV in "Happy Face Murders."

CAPTION: Marg Helgenberger in "Happy Face Murders."