Unlike so many biographical dramas that wrap up their subjects in neat little packages and apply labels for easy identification of the contents, "Breaking the Code" is apt to leave you with furrowed brow.

Its central character, British mathematician Alan Turing, was largely responsible for cracking the Nazis' "Enigma" code during the early years of World War II. But in the play Hugh Whitemore has fashioned out of Turing's controversial life, he is something of an enigma himself.

The play, written specifically for British actor Derek Jacobi, opened a four-week pre-Broadway tryout at the Eisenhower Theater Saturday night, and there's no doubt that Jacobi is vivid in the role. If the performer, who last graced the Kennedy Center with the extraordinary panache of Cyrano de Bergerac, wanted a change of pace, he couldn't have asked for more.

As Turing, he chews his nails, fidgets on dowdy furniture and toys with his shoelaces. His speech is punctuated with a horrible stammer, which distorts his features and makes him look periodically like a goldfish plucked from its bowl. He has the mark of the unapologetic sloven, and his tie, when he wears one, invariably protrudes below his sweater and hangs over his belt. The brilliance of the genius is his, but so is the impetuousness of the schoolboy. And he is a homosexual.

That Jacobi gives a performance of riveting details does not solve the evening's problems, however. "Breaking the Code" never puts the man solidly within our reach. He remains an exhibit in a cerebral sideshow, a fascinating oddity who probed so deeply into mathematics that he was really dissecting the very nature of the human mind.

Far from the least of the puzzlements he posed was his suicide in 1954 at age 41, occasioned by the eating of an apple, laced with cyanide. (The apple of knowledge, perhaps?) Was this the desperate gesture of a hounded man, forced by the authorities to take female hormones, which in turn caused him to develop a woman's breasts? Or was it the final bold experiment of the scientist, who wanted to discover if the mind had an existence independent of the body?

Whitemore has given himself some huge challenges in dramatizing this walking conundrum. On the one hand, he wants to portray the depth of Turing's intellect and the intensity with which he attacked mathematical complexities and paradoxes beyond the ken of most of us. Genius is perhaps the most difficult quality to render on the stage. To simplify it for common consumption is to betray it. And yet to explore it fully is to risk leaving the audience in the dust.

Second problem: Much of Turing's private life could be easily construed as the stuff of tabloid journalism. And yet, he seems to have been neither a wildly tormented individual nor an early crusader for gay rights. He had a rather forthright attitude toward his sexuality and in all blue-eyed candor told the police about the affair for which he was eventually prosecuted and found guilty. If anything, he believed that the same open and unprejudiced mind he brought to scientific research should bear on matters of morality.

Consequently, Whitemore is obliged to deal coolly with a hot issue, to pull back whenever sensationalism threatens to rear its head and to spurn the temptation to make a martyr of Turing. Under the play's surface, you can sense considerable back-and-forth tugging.

"Breaking the Code" begins with the police grilling Turing, whose apartment has just been burglarized, and then flashes back in time to sketch his family life, his school days and an early platonic love affair, and his breakthrough work with British intelligence during the war. Regularly, however, Whitemore returns to the police investigation, which is getting ever closer to uncovering Turing's sexual proclivities.

As the play is structured, we are led to believe that Turing's trial for "gross indecency" is going to be the linchpin. Surely, we are building up to a big moment, when a man's soul is suddenly on the line and he rises to the occasion or is destroyed by it. No such thing. "Breaking the Code" vaults over the trial and its consequences to show us Turing, many years later, after he has digested the affronts and indignities, and has resumed, for better or worse, his delvings into computer technology.

As you might expect, the contradictory impulses take their toll. Despite Jacobi's restlessly alert performance, the play never delivers a knockout punch and ends up seeming rather dry. The quality of its ideas is far greater than the emotions it stirs. On a holiday in Greece, Turing admits to a Greek pickup that he has had trouble all his life integrating thought and feeling. It is a fair diagnosis of the play itself.

One certainly can't fault the production, which has been directed unflinchingly by Clifford Williams. There is not a lot of warmth in the proceedings, but the chilliness is somewhat abated by the elegant Rachel Gurney, as Turing's mother, a woman who understood little about her son's life, but who, in the play's most moving scene, understands enough to accept his homosexuality. Colm Meaney has the dull inflexibility of the police sergeant, chipping away at Turing until Turing tells him what he wants to hear.

There are sharply etched performances, as well, by Michael Dolan, the lower-class greaser in Turing's bed; by Jenny Agutter, as the coworker on the "Enigma" project who would have married Turing, had he been willing to compromise for appearance's sake; and by Michael Gough, the paternal and quietly sagacious intelligence chief, who tries to persuade Turing that discretion need not necessarily be the lesser part of valor.

The action takes place in a large dark box, designed by Liz da Costa to resemble the inside of a mechanical brain as Hollywood might have depicted it in an old horror flick about a mad scientist. Now and then, however, the upstage panels part to reveal great banks of clouds. The effect is both arresting and ironic -- the billowing clouds suggesting the endlessness of eternity and the breadth of human thought, which may be one and the same.

This was where Turing felt alive, vital and pure. Human society tripped him up. He could intuit dazzling scientific theories, but he failed to understand that in daily commerce truth is a dangerous thing. Numbers, Whitemore has him say at one point, were his best friends when he was growing up; people eluded him to the end.

"Breaking the Code" is an ambitious, questing play. But I'm not sure it knows entirely what to make of Turing. As a result, neither do we.

Breaking the Code, by Hugh Whitemore. Directed by Clifford Williams; scenery and costumes, Liz da Costa; lighting, Natasha Katz. With Derek Jacobi, Rachel Gurney, Jenny Agutter, Colm Meaney, Michael Gough. At the Eisenhower Theater through Oct. 31.