ON OCTOBER 5, the first Monday in October, Sandra Day O'Connor will formally become the first female justice of the United States Supreme Court. Perhaps, on this historic occasion, you find yourself wondering how the other eight brethren will react to their new sister, and this curiosity could even inspire you to see The First Monday in October, a movie ostensibly based on this very question. Before you do, consider the following evidence.

The screenplay, adapted by Jerome Lawrence and Robert E. Lee from their play of the same name, sounds like a third-grade primer on Constitutional Law, replete with metaphors for an eight-year-old. "You can't turn the law into a straightjacket," feisty liberal Justice Dan Snow (Walter Matthau) tells arch-conservative bench-mate Ruth Loomis (Jill Clayburgh). "It must be a suit of clothes you can move around in." With this profound thought as a guideline, the movie dashes madly from issue to issue, like a tourist with an hour to spend on all of the national monuments. But the camera never wanders too far from Matthau and Clayburgh, as they duke it out over everything from dirty films to women's rights.

The opening round concerns the First Amendment rights of the director of The Naked Nyphomaniac, an "educational documentary," the defendant claims, about the sad life of Lois LaRue, "the woman who just couldn't get enough." Predictably--as is everything else in First Monday-- Justice Loomis upholds the conservative cause against the biting rhetoric of Justice Snow, who boldly declares that "one man's pornography is another man's poetry" and vows to "defend every man's right to be wrong."

The battle never reaches its climax, for as soon as the bare breasts of Lois LaRue have disappeared from the courtroom screen, we are launched into Snow's private life and his problems with a wife who walks out because he can't remember the color of the wallpaper in the living room. By this time you will have thought of several more convincing reasons to leave Snow, but director Richard Neame wisely does not dwell on these, bustling back to Capitol Hill, where Snow (rarely one to sulk) is passionately trying to convince Loomis and the rest of the gang to hear a case against Omnitech, a multinational corporation whose president has allegedly made off with patents to a new gyroscope engine. Snow and Loomis argue.

Before First Monday ends, a mere 45 minutes later, Loomis has turned jurist-detective, implicated a long-time boyfriend in a major political scandal and developed something more than a good working relationship with her former foe, Justice Snow.

MATTHAU AND CLAYBURGH try to sneak through this barrelling plot, hoping that if they just read their lines, no one will notice that they are even in the movie. As usual, Matthau lets his flabby-cheeked scowl and floppy jackets substitute for any real characterization. He makes it clear that Snow has a messy desk, a messy marriage and a mess manner--and he lets it go at that.

Clayburgh lacks whatever rumpled charm Matthau manages to muster. She builds her few funny lines into heavy-handed moral treatises that collapse under their own weight.

To be fair, the actors get no help from their director. Neame paces the film by putting a five-second pause between each line of dialogue, as if he were going to splice in an audience laugh track but forgot at the last minute. The dead time only further highlights the inanities Matthau and Clayburgh spit at each other. In an effort to create the stately atmosphere of the high court, Neame relies almost exclusively on close-up, static shots of the two principals inside their chambers. Without any camera movement, he creates a Bergman-like claustrophobia--ridiculously out of place in this comedy.

Finally, instead of concluding the mess, Neame merely returns to the writers' tired mix of bland humor and semi-meaningful moralizing. Loomis and Snow, now buddies, enter the court, grinning. She whispers, "You and I make each other possible." Matthau seems too bored to respond.