THE MIND of the playwright is not unlike that of the master criminal. Before acting, each must construct a well-conceived, meticulous plan, tightly bound together, without any loose ends. In the case of Softly Stealing, the new Kirkland House musical, the mastermind is that of Tom Fuller '74 of Harvard's Gilbert and Sullivan fame, and the plot rocks with enough surprising twists and turns to be worthy of the reputation of Edward Sable, its notorious but good-hearted Victorian robber hero.

In Sable's charmingly deluding scheme, which becomes, of course, Fuller's wonderfully beguiling play, actor and crook are one. Posing as a family friend who is at home in England for the first time since he was seven, Sable goes to the country estate of Lady Beatrice Harlston to steal some of her plentiful jewels. Take the goods and run was the direction given to Sable by Harry Mercer, his surly subordinate, and Brenda, his impatient moll. But Sable overstays his visit, tempted to steal not the jewels but the suspicious and susceptible heart of the Lady's daughter Julia. Fuller's original songs about the crime that may or may not be committed are so thoroughly entertaining that Sable's dangling at the end of Lady Beatrice's necklaces becomes nothing less than enjoyable and Softly Stealing a cause not for anguish but for delight.

Just as in a successful crime scheme, it is the well-wrought details that enable the perpetrator to get away cleanly, so in Softly Stealing it is the lyrics to the 19 songs that provide the great escape. Fuller's words can be alternately funny, as they are in "Taxing Deductions," the theme song of the "almost clever criminologist," Inspector Quentin Thornblade, who tries to think like the great Sable in an effort to outwit his criminal mind, or haunting as in "The Runaways," Brenda's plea to Sable to return home, or romantic as in "A Perfect Stranger," the love song in the play. But all the lyrics are so perfectly set to the music that the seem to have always belonged there, like the jewels that stud Lady Beatrice's pendants.

Much of the credit for this achievement should go to Fuller who, by directing his sophisticated, well-constructed piece with the accent on style and what he terms "the very, very small things," like inflection and timing, has put the final polish on the play himself. But a big portion of the applause should also go to Gerald Moshell, the Kirkland House music tutor and Fuller's partner in this crime, who has composed a score so suited to the lyrics in mood and meter that, as Fuller himself has said, "the words and music seem to have something to say to each other." When the words say "my song must adroitly change its key," and the music does, as happens in "Chaucer's Lament," it is only the most obvious working together that is everywhere between script and score. Moshell's music unravels from soft, lush chords, reminiscent of Ravel or Poulenc, to what sounds like the throaty, brassy tones of Kurt Weill with the ease of the plot itself.

In crime, as in theater, it's not just a masterful conception but also a flawless execution that counts. Unfortunately for Softly Stealing, staged in the Kirkland House Junior Common Room with a case composed almost entirely of students from the House, the execution is not always quite right. The main problem is that while the interdependence of words and music is one of the major triumphs of the play, the tendency of the less experienced members of the cast to rely on the others clearly is not.

Fuller, who a week ago stepped in to replace an injured actor, plays Sable with the strong tenor and swashbuckling panache which fans of this now author and director in addition to Gilbert and Sullivan-trained actor have come to love and expect.

Almost equally good, however, are Sable's partners and opponents in crime. As Brenda, Patty Woo moves gracefully from the woman abandoned by her man to the tough Mae Westish cookie who is determined to take her revenge. When Sable introduces her to the Harlstons as a student at a finishing school near Oxford, the jealous Brenda snaps "But I'm not finished yet." Larry Schneider's gutsy characterization of Harry Mercer is not quite so well developed as Woo's, but with his hardbitten face and sly movements, Schneider fits the image of the life of crime personified.

With his fine singing voice and utter disdain for everyone, Rick Farrar as Chaucer, the butler who did, did not, and did do it, almost succeeds in making his share of the crime perfect. And Harry Dorfman, as the inspector who wishes the criminals in his district would be "a bit more industrious and inventive," turns in what Thornblade, mispronouncing his favorite adjective, would call a "consummate" performance. Dorfman's moments at the police station with his amiable sidekick Grover Bagby, well played by Martin Marks, almost (forgive the expression) steal the show.

AMONG THE RICH and would-be wealthy (who really are not used to this kind of thing, you know) there are some near headaches. As Lady Beatrice, Bonnie Landers lacks sufficient crustiness for a member of England's upper class. Landers, who like most of the rest of the cast has a good voice, repeatedly changes her character and looks as if she is surprised to be there every time she walks on stage. As an actress, Meredith Birdsall as Julia, the overprotected daughter who "could live 20 more years and still not be a normal girl of 21," is somewhat better. But Birdsall is too nervous on stage to feel comfortable with the role of the girl who doesn't feel quite comfortable with Sable.

Similarly, Frances Gross, in her portrayal of Grover Bagby's shrewish wife Cora who deduces what's really going on, shows a little too much shrew. And the four maids who comprise the play's chorus are too disorganized in their organized disorganization for their own good. Only Vivian Cavalieri, the first maid, could do anything so practical as sing completely, and quite well, on tune.

In the second act of Stealing, this chaos of the maids seems to rule too often. The act's second song, "He's a crook!", which is sung by the maids, denies what we have just seen two scenes earlier. The major problem in the act however--slightly underrehearsed but good choreographed numbers--can be easily fixed if the show takes a well-deserved extension of its run next weekend.

"Every crime is a masterpiece," Grover Bagby tells Chaucer when that poor butler is in jail. With its neat plot, irrepressible music, and at times brilliant lyrics, Softly Stealing is not quite a masterpiece. But at times this original crime story seems to come very, very close.