A PARTRIDGE IN A PEAR TREE by Leslie Stevens; directed by Philip Abbott; settings by Robert Randolph; with James Mason, Clarissa Kaye, Ivor Barry, Don Draper, Gerald S. Peters, Martha McFarland, Elizabeth Hansen, Charles Knox Robinson, June Barrett and Robert Nadder.

At the Eisenhower Theater through Jan. 28.

A rare opportunity awaits those who rush right out to see "A Partridge in a Pear Tree." Too often, the public is allowed into a play only when the actors have learned their lines and the director has found his rhythm and the playwright has plugged the holes in his plot. Such evenings can be pleasant after their fashion, but they provide a deceptively sketchy impression of what the theater is about. The theater is about pain and woe and grief and trial and error and more error.

Hence the bold decision by the producers of "A Partridge in a Pear Tree" at the Eisenhower Theater to let us watch James Mason and Clarissa Kaye (Mrs. Mason) in rehearsals of this new play -- at no extra cost beyond the usual price of a finished performance. The producers did not actually advertise last night's opening as a rehearsal, but in view of Mason's inability to get through more than a few speeches in a row without a lapse of memory, and the absurdly schlocky, caricatured performances of virtually the entire supporting cast, and the general absence of anything resembling direction, that is what I choose to believe.

The author of "A Partridge in a Pear Tree," Leslie Stevens, has been toiling in television-land for many of the years that have elapsed since his big stage hit of the '50s, "The Marriage-Go-Round." And on the preliminary evidence of this play, his mind seems to have been adversely affected by the experience. "Partridge" has the look of an ABC "Mystery of the Week."

It begins at the end of a murder trial -- the trial of a London housekeeper (Kaye) for a man's hit-and-run murder. The housekeeper, a coarse Australian-born woman, pleads guilty and won't hear of being defended. But the conscientious judge (Mason) summons her to his chambers for some last-minute in-camera interrogation.

They are acquainted, it develops. In fact, they were lovers long ago and far away, and the judge knows the defendant well enough to know she could never have committed the cold-blooded murder of which she stands accused. In tiny bits and pieces, she admits there is far more to the tale than the trial testimony has revealed, and gradually it spirals into a colossal scandal. The judge's task, with assistance from his buffoon-like counsels for the prosecution and defense, is to take all the complications and "stuff them back into the teddy" --i.e., find a way to do some approximation of justice and protect as many reputations as possible.

It is hard to judge a play's possibilities from such an awkward and tentative performance. Nevertheless, there are a few unpromising constants here, and one of them is the underlying premise. The judge's dilemma is whether to diqualify himself from ruling on the fate of a defendant he knows. By the end of the trial, that has certainly become a ticklish issue -- for reasons it would be indelicate to reveal -- but the judge never explains why he didn't simply disqualify himself at the outset.

Such queries would be irrelevant, needless to say, if "A Partridge in a Pear Tree" were the laugh riot it so dearly aspires to be. But mixing mystery and comedy is a delicate art, and none of the principal parties to this venture shows much feel for it. Mason, in particular, has witty lines that get thoroughly lost in his struggle to remember who's who and what's what -- and stiff-upper-lip comedy is hardly his forte in any case.

This is one of those "lie-within-a-lie-within-a-lie" puzzle plays that aims to evoke happy memories of "Sleuth" and "Deathtrap" and "Witness for the Prosecution." But for Washington audiences, the obvious comparison -- unfavorable at that -- is with "Tricks of the Trade," another petrified trifle of a suspense play with another husband-and-wife starring duo, George C. Scott and Trish van Devere. When will it all end? The suspense is killing us.