Arcadia is the most perfect, the most intricate, the most moving of Sir Tom Stoppard's plays. It is like a sad waltz and a ballet, it is about thermodynamics, literary criticism, the history of gardening, and the nature of truth, and it is very funny.

Where to begin? There are so many interweaving threads. It is set at Croom Park, a stately home, in 1809 and in the present day. Thomasina is a 13-year-old girl, and Septimus is a 22-year-old who is her tutor in Latin, mathematics, and music.

THOMASINA: Septimus, what is carnal embrace?
SEPTIMUS: Carnal embrace is the practice of throwing one's arms around a side of beef.
THOMASINA: Is that all?
SEPTIMUS: No... a shoulder of mutton, a haunch of venison, well hugged, an embrace of grouse... caro, carnis; feminine; flesh.
THOMASINA: Is it a sin?
SEPTIMUS: Not necessarily, my lady, but when carnal embrace is sinful it is a sin of the flesh, QED. We had caro in our Gallic Wars -- 'The Britons live on milk and meat' -- 'lacte et carne vivunt'. I am sorry that the seed fell on stony ground.
THOMASINA: That was the sin of Onan, wasn't it, Septimus?
SEPTIMUS: Yes. He was giving his brother's wife a Latin lesson and she was hardly the wiser after it than before. I thought you were finding a proof for Fermat's last theorem.
THOMASINA: It is very difficult, Septimus. You will have to show me how.
SEPTIMUS: If I knew how, there would be no need to ask you. Fermat's last theorem has kept people busy for a hundred and fifty years, and I hoped it would keep you busy long enough for me to read Mr Chater's poem in praise of love with only the distraction of its own absurdities.
THOMASINA: Our Mr Chater has written a poem?
SEPTIMUS: He believes he has written a poem, yes. I can see that there might be more carnality in your algebra than in Mr Chater's 'Couch of Eros'.
THOMASINA: Oh, it was not my algebra. I heard Jellaby telling cook that Mrs Chater was discovered in carnal embrace in the gazebo.
SEPTIMUS: (Pause) Really? With whom, did Jellaby happen to say?
So it begins. Thomasina Coverley, daughter of the house, is a genius; she is so good at mathematics that she is unutterably bored with the euclidean geometry that Septimus is trying to teach her, and in the course of the play she gropes towards chaos theory, fractals, recursion, concepts that would not become known for 150 years. Septimus is screwing the wife of Ezra Chater, the resident bad poet. He is also attempting to review Chater's garbage for a journal.

Croom Park was surrounded by Italianate classical gardens until about 1750, then Capability Brown landscaped it to a lawn with hahas, and now (1809) the landscape gardener Noakes is planning to turn it into a wild, rugged, romantic never-never land, with a hermit's cottage picturesquely situated, because it is the modern fashion.

In our modern era, Croom Park is visited by two literary people. Hannah Jarvis is working on a history of the garden, and is particularly interested in the mad hermit who lived there for many years. Her previous book was on "Caro", Lady Caroline Lamb, Byron's one-time lover; but it was a popular book, so the academic crowd of Byron experts pissed all over it.

Enter Bernard Nightingale, an academic Byron expert whose review of Hannah's book was particularly acerbic. He has new evidence that links Byron with Septimus Hodge the tutor and Ezra Chater the poet in a startling theory that will make them famous, but he needs Hannah's cooperation.

Also in the present day, Valentine Coverley, the young man who will be heir to Croom Park, is a mathematical biologist studying recursive equations by using the population genetics records he can get from his own family's grouse-shooting books, going right back to the time when Septimus and his school-chum Byron were here.

When confronted with evidence that his long-dead kinswoman, the child Thomasina, had discovered the advanced mathematics he is using, Valentine refuses to believe. Bernard and Hannah also argue over whether Byron was involved in a duel, and whether the documentary evidence is enough to prove it. Because we are seeing both ages interweave on stage, there is dramatic irony when we can see part of what is wrong with their positions and see them partly vindicated.

Arcadia premièred at the Lyttelton Theatre, at the National Theatre in London, on 13 April 1993. Emma Fielding was Thomasina, Felicity Kendal was Hannah, Bill Nighy was Bernard, Samuel West was Valentine, and Harriet Walter was Lady Croom. I have seldom been so moved by any piece of theatre. Everything in it fits together so perfectly, all the different threads dance together, all the meanings and ironies and tragedies intersect.