Who is that sleek young time bomb of a man? At moments, he seems as frightened and helpless as a 4-year-old who has lost his parents at the fair; at other times, he is the soul of patrician urbanity, your ideal white-tie dinner partner. Then the eruptions come in nasty bolts of anger. And they aren't half as scary as when his face freezes into a grim, affectless mask. What is he called again? Oh, right. Nero.
Yes, that Nero, the Roman Emperor whose name has become a byword for cruel and unusual despotism, and who, as embodied by Toby Stephens, is holding compellingly tormented sway over the Majestic Theater at the Brooklyn Academy of Music in the second of two imports from the Almeida Theater of London.
It isn't the familiar full-blown tyrant with violin that Mr. Stephens is portraying in the crackling production of Jean Racine's ''Britannicus'' that opened on Friday night. It's something far more intriguing: a man-boy on the cusp of monsterhood, someone who, in the course of the evening, will take that last step into a state beyond moral redemption.
You don't have to look far to see what propels him there: it's Mom, of course, the scheming, power-hungry Agrippina, and she is played to a savory fare-thee-well by Diana Rigg. This fine actress would have seemed to have provided the last word on strangulating maternal figures in her sublimely creepy performance in the television drama ''Mother Love.''
Here, however, she offers an equally forceful portrait that is also much more nuanced. Under the direction of Jonathan Kent, Ms. Rigg and Mr. Stephens perform a mother-and-son duet that works both as enjoyable political soap opera and unsettling psychological portraiture, kindling the histrionic sparks that are oddly absent from their teamwork in Racine's ''Phedre,'' which runs in repertory with ''Britannicus'' through Jan. 17.
It's a shame that ''Britannicus'' has been allotted only 4 performances (the remaining two are on Thursday and Friday) to ''Phedre's'' 10 during the Almeida's visit to the academy. You can understand the logic. ''Phedre'' is a known quantity, at least to anyone who took introductory French literature, and the title role, essayed here by Ms. Rigg, has been a testing ground for great actresses for centuries.
Yet the Almeida's ''Phedre'' is crippled by a palpable awareness on the part of its creators that they're tackling something big, with an attendant stiffness that isn't entirely offset by Ms. Rigg's viscera-twisting performance. In contrast, all involved in this ''Britannicus'' exude a relaxed, pleasurable confidence. They're having a fine old time. Undoubtedly you will, too, while getting to know, under the best possible circumstances, a play that doesn't deserve its obscurity among English-speaking audiences.
What a play it is: a piano-wire-taut piece of work that obeys the Aristotelian unities while opening windows onto all sorts of levels of contemplation. Although it deals with the corrosive effects of obsessive love found throughout Racine, it is far less monolithic than usual in its approach. ''Phedre,'' as the poet Paul Valery noted, turns into a monologue in the memory. ''Britannicus'' is a concert of voices vying for dominance, none of which are as one-note as you might suspect.
It's as entertaining a tale of Roman decadence and cut-throat politics as Robert Graves's ''I, Claudius,'' but it resists turning its characters into mere players in a sinister chess game of good and evil. The motives of the power jockeys in ''Britannicus'' aren't entirely clear even to themselves. Alliances are untrustworthy precisely because human personality is so fluid. Anyone following the current breakneck reversals in the real-life drama of the American Presidency will have no difficulty giving credence to the ever-shifting world of ''Britannicus.''
It was Racine's great inspiration, in reshaping accounts from the annals of Tacitus, to present a fledgling emperor who had not yet become the eponymous Nero of legend. The 22-year-old has ruled Rome for only six months when the play opens, and he is just starting to bristle under the control of Agrippina, whose machinations landed him on the throne at the expense of his rival and stepbrother, Britannicus (Kevin McKidd).
Nero especially resents his mother's having arranged the engagement of Brittanicus to the chaste and exquisite Junia (Joanna Roth), and has had the girl made a prisoner in the imperial palace. This political move is complicated by the Emperor's response on having seen Junia for the first time. As he tells one of his tutors, Narcissus (Julian Glover), ''Nero is in love.''
Mr. Stephens does a lot with that simple line, inflecting it with a combination of regal arrogance and embarrassment. He and Mr. Kent consistently and effectively emphasize Racine's idea of Nero at a crossroads. His divided potential, to become an honorable leader and a self-indulgent tyrant, are embodied by his two advisers, the canny but virtuous Burrus (David Bradley) and the devious Narcissus, both persuasively portrayed.
Yet there is, from the outset, an unhinged quality to this Nero that the intoxication of power is pushing toward sadism. It's all in the text, but Mr. Stephens taps it with flair, as in his quivering description of his first vision of Junia. What clearly attracts him, more than anything else, is her state of distress. And you are always aware of his profound ambivalence toward his mother, to whom he remains, to his shame, infinitely susceptible.
Ms. Rigg has chosen to play Agrippina not as a vengeful serpent but as the ultimate politician, a brisk, unflinching pragmatist with an outsize ego, and the decision works wonderfully. There's a touch of both Margaret Thatcher and Hillary Rodham Clinton to her Roman matron, clad in Maria Bjornson's Chanel-like ensembles.
This is someone who has mastered the rules of the game and knows just when to intimidate and when to cajole, plying her majestic alto voice in ways that brook no argument. She accomplishes the considerable trick of showing when she is wounded without ever displaying signs of weakness. For all her self-admiration, she is not a deluded woman, either, and Ms. Rigg makes it believable when Agrippina becomes the voice of the conscience of history, which will condemn her son in the centuries to come.
The dead-on look of the show by Ms. Bjornson and the lighting designer, Mark Henderson, suggests an Italian Fascist version of the corridors of Buckingham Palace. Ms. Bjornson and Mr. Kent make terrific use of a set of giant fish tanks, metaphors for both entrapment and the dauntingly public nature of its characters' lives. In the evening's most potent image, Nero hides behind one of those tanks, his face visible to the audience through the water as a melting mask, as he watches an encounter between Junia and Britannicus.
As the naive victims of a corps of arch manipulators, Ms. Roth and Mr. McKidd have thankless roles, but they play them with an energetic agitation that makes their inevitable fates the more tragic. And Barbara Jefford, looking like an elegant Miss Marple as Agrippina's confidante, is a nicely restrained foil to Ms. Rigg's grandeur while clearly cut from the same cynical cloth. All of the cast, for the record, is utterly at ease with Robert David MacDonald's first-rate rhymed translation, its artificiality coming to feel like the natural language of a courtly society.
As in much of Racine, the most spectacularly dramatic moments take place offstage, but you never feel cheated by only hearing about them. Mr. Kent creates the tense, calculating atmosphere of candidates at a Presidential convention waiting in a private chamber as their fates are decided outside. There is a regal staircase, revealed only in the play's second half, at the back of the stage, with steps leading to an unseen room where a murder takes place that will change the course of an empire.
By the play's end, that room, we are told, has become a scene of grotesque chaos. Agrippina's response to the description of that scene, as impeccably rendered by Ms. Rigg, says everything about the world she inhabits. She is appalled and, in some way, broken, but there is also some cold internal mechanism that clicks on and allows her to start assessing the damage and think about tidying up. It is, in other words, politics as usual.