What was good about "They're Playing Our Song" when it opened on Broadway a few seasons back is precisely what was good about "They're Playing Our Song" when it opened last night at the Warner Theatre: Lucie Arnaz.

In a jerry-built musical that stretches its modest story to dismaying lengths and coughs up one-liners with the grace of a backfiring jalopy, she remains a human being--vulnerable, appealing and real. Not musical-comedy real, but off-the-street real. True, you might never bump into her at an intersection, but she leaves you wishing you would. Although the character she plays--a young lyricist named Sonia Walsk--is a certifiable kook, Arnaz is not about to run away with the quirks and eccentricities, quaint as they may be.

All around her, "They're Playing Our Song" is making the most obvious ploys to amuse and entertain (the book is a mere indulgence by Neil Simon), but Arnaz is trying to show us what makes Sonia tick. When she sings, which she does with a pleasant plaintiveness, it's not just because songs are there to be sung. It's because she wants to open up her heart a little more.

Arnaz constitutes half of the cast of this two-character musical. The other is Laurence Luckinbill, as a wunderkind composer named Vernon Gersch. The evening's tribulations are purported to be inspired by the professional and personal relationship between composer Marvin Hamlisch and lyricist Carole Bayer Sager. Hamlisch, in fact, provided some attractive tunes for "Song," and Sager contributed the contemporary lyrics. But whatever else went on in their life together is barely touched upon here. Despite the "now" trappings, the story is standard sitcom--flaky girl meets neurotic boy, loses neurotic boy, wins neurotic boy.

Except that Luckinbill is not exactly a boy these days. He's really not much of a musical comedy performer, either. He is, however, the husband of Lucie Arnaz, and while they must relish the opportunity to work together nightly, the pairing doesn't make for much on-stage magic. He looks rather like a nutty professor, with his chubby cheeks, his thinning hair combed forward and a slightly manic glint in his eye, and he virtually pounces on every joke, as if he hoped to divert attention from the fact that he is, all in all, rather sadly miscast. That his singing voice is perfectly undistinguished is perhaps less important than its tendency to stray off key. (Far closer to the mark was the elfin charm of Victor Garber, when "Song" played the National Theater two seasons ago.)

Of course, "Song" makes efforts to convince us that it is fuller than it really is. The sets, enhanced by some flashy projections, carry on more than most. There are six briefly seen chorus members, who function as alter egos to the principals. And an offstage character named Leon, Sonia's former boyfriend, telephones her frequently and occupies a lot of discussion.

But in no time, "Song" comes back to its two lovers and the little roadblocks life (or Simon) keeps erecting before them on their way to the final clinch. I certainly wanted Arnaz to get her man. Unfortunately, Luckinbill was not the man I wanted her to get.

THEY'RE PLAYING OUR SONG. Book by Neil Simon. Music by Marvin Hamlisch. Lyrics by Carole Bayer Sager. Directed by David Taylor. Musical staging, Lani Sundsten. With Lucie Arnaz and Laurence Luckinbill. At the Warner Theatre through Aug. 8.