In 'The Real Thing,' Tom Stoppard tackles reality in a fictional world

With an almost all-new cast firmly in place, ''The Real Thing,'' at the Plymouth Theatre, continues to be one of the best-acted plays in New York. In terms of stimulating ideas, the prize-winning Tom Stoppard comedy about love, language, and the meaning of commitment would rank high in any season. It is the most eloquently articulate example of Mr. Stoppard's gift for cerebral comedy - in this case cerebral comedy with a heart.

The lively conflict animating ''The Real Thing'' centers on Henry (John Vickery), a playwright, and Annie (Caroline Lagerfelt), an actress. Their brief extra-marital affair leads to marriage, following their respective divorces.

The action begins with a scene from Henry's reigning London hit, ''House of Cards,'' a comedy about supposed marital infidelity. By retaining the two actors from the opening scene in the developments that follow, Mr. Stoppard plays a slightly Pirandellian game, as he considers just what ''the real thing'' really is.

The play-within-a-play is, however, merely the launching pad. The question Mr. Stoppard poses - and hintingly answers - is whether there is enough real love and understanding in the Henry-Annie relationship to overcome their differences on a variety of matters, from music and language to activist politics.

Henry, a master of the barbed small talk of social comedy, fashions commercially successful entertainments (reminiscent of Noel Coward). Annie has espoused the cause of the antinuclear crusade and in particular of a young Scottish protester named Brodie (Vyto Ruginis). While absent without leave from the Army, Brodie followed Annie into a demonstration, in the course of which he set fire to the wreath at the foot of the cenotaph honoring Britain's war dead.

The action earned Brodie a six-year prison sentence. While in jail, he has written a left-wing diatribe of a play which Annie would like to do on television. Her hope is to call attention to the half-forgotten prisoner and perhaps help win his early release. Will Henry rework Brodie's unwieldy script to make it presentable?

To the impulsive and susceptible Annie, Brodie is ''a prisoner shouting over the wall.'' She insists that what the young man has to say is more important than the naive clumsiness with which he says it. To Henry, Brodie is ''a pacifist hooligan'' and, perhaps worse still, ''a lout with language.'' Says Henry: ''He can't write. He's got something to say. It happens to be something silly and bigoted. But leaving that aside, there is still the problem that he can't write.''

In some of his best lines, cricket enthusiast Stoppard uses the image of the ''cunningly put together'' cricket bat to illustrate what a writer is trying to do - ''to write cricket bats, so that when we throw up an idea and give it a little knock, it might ... travel ... I don't think writers are sacred, but words are. They deserve respect. If you get the right ones in the right order, you might nudge the world a little or make a poem that children will speak for you when you are dead.''

Nevertheless, for the love of Annie, Henry rewrites and reshapes Brodie's play. Annie appears in it on TV, and the prisoner wins a reprieve. Although the denoument of ''The Real Thing'' exposes Brodie as a worshipful fan of Annie's rather than as the martyr to a cause, Mr. Stoppard has scored a number of points. He has made them with wit, eloquence, and an emotional thrust that can be very moving.

''The Real Thing'' ranges widely and brilliantly. Its devices extend from that opening scene, which throws the audience slightly off balance, to illuminating excerpts from Strindberg, the Elizabethan John Ford, and even the clunking Brodie. These are not merely clever interpolations and allusions. They are skillfully integrated elements of a work that constantly demonstrates how ''the real thing'' - the themes and ideas of a play - can exist in a make-believe world.

Meanwhile, Mr. Stoppard scores neatly with his writer's cricket bat. Besides the sometimes fragile nature of fidelity, ''The Real Thing'' deals wittily with an agenda that includes parent-child relationships in a ''liberated'' society, intellectual playwrights like Henry/Stoppard, the British class system, the ephemeral nature of causes, and the dangers of sloganeering - the notion that there's ''no philosophy that can't be printed on a T-shirt.'' Most of all, of course, the play about love.

''It's no trick loving somebody at their best,'' remarks Henry. ''Love is loving them at their worst.''

Under Mike Nichols's seamless direction, ''The Real Thing'' unfolds with cinematic fluidity through its 12 scenes (in two acts). The production is technically superb. Everything about it contributes to the exhilaration of sharing an experience in which lively intelligence and creative talent dedicate themselves to a play of ideas. That's the real thing!