TAKING the first capital letter of pages 36, 51, 196, 205 and 222 (the mathematical logic will be obvious to the adept) yields the mysterious sequence S, H, I, T and E. The significance of this remains unclear, though I have discovered that my own copy falls open at exactly the same angle as the Great Pyramid of Giza, so we may be on to something.

God protect us from more Da Vinci Code-style nonsense like this. The 'plot' concerns Father Agostino Leyre, a queasy member of the Holy Office more interested in textual matters than the fire-andpincers approach of the Inquisition. He is sent by Pope Alexander VI, who believed he was descended from Osiris, to supervise the final stages of Leonardo's great but mysterious masterpiece, 'The Last Supper'. The painting is strange in that it shows neither Grail nor Eucharist, but apparently the moment when Jesus announces he is betrayed, to varying reactions from the disciples. A trio at the far right have turned their backs on Christ and are locked in conversation.

Leonardo is using members of the Dominican order as models, but may have used a female face for John, also known as Mysticus, ?He Who Knows the Mystery". He seems also to have Plato (thrown overboard by the Roman Church in favour of Aristotle) as Simon Confector, the focus of that inattentive group at the right, and it's definitely Leonardo himself as Judas Thaddeus.

His role as Occultator is ?He Who Conceals", and it's widely thought the artist, who seems to have adopted some of the old Cathar heresy, buried a powerful and subversive message in his greatest work.

There's a warrant for the suggestion. It was thought the last of the ?bonshommes" had been immolated at Montsegur in 1244, but many Cathar families fled into Lombardy and preserved elements of their doctrinal purity for the next 250 years. Leonardo may well have had access to gnostic texts.

Nobody will be surprised to hear that the Mary Magdalene bloodline plays a part.

But . . . and it's a substantial but . . . the theory needs a more secure narrative scaffolding than it gets here.

Some of the monks involved in the painting are being bumped off, and Leyre is caught up in a cat-and-mouse pursuit of the mysterious Soothsayer. His only guide is a short text in doggy Latin and with strange dots above certain letters, which nobody seems to notice for the next 100 pages. Though normally the most passive of mystery readers, I cracked it on page 244, which left 70 wearying pages to be played out.

Sierra has a reputation as the author of mysteries based on scientific and historical enigmas and, to be fair, La Cena Secreta was published in Spain before The Da Vinci Code became the phenomenon it is. Translator Alberto Manguel also has a reputation as a novelist and literary historian, so it's hard to believe he accepted a book so shoddily written and then left it in such a drab state, full of syntactical lapses. He couldn't do much about the plot or dialogue, which creaks like an unoiled church door, but he might have left it in English.

The best guess is that those mysterious letters refer to the Hesti, an ancient tribe of, er, tribesmen, famed for their occult knowledge of anagrams and apparently the first people in history to have developed high-concept bandwagon publishing. Probably.