Antony Sher plays around with history in The Giant and dreams up an erotic fantasy. His fantasising depends upon the impact of suppressed homosexual desire on two great Renaissance artists, Michelangelo and Leonardo da Vinci, who are stirred to steaming point by a young quarryman with luxurious pubic hair and other cherished accoutrements.

In real life the two men met just briefly in the street. Here they have several encounters: they compete for the commission to sculpt David's statue and quarrel over Vito whose well-honed body is used by John Light's surly, hang-dog Michelangelo as model for his sculpture. If Sher's speculative and ambitious adventuring in the worlds of 16th century Florence, power politics, artistic creativity and sexual psychology had proved as interesting and purposeful in theatrical practice as they sound in outline, The Giant might not be such a confused, unrewarding experience.

Having settled upon his invented dramatic scenario, Sher seems prepared to demonstrate how gay impulses inspired, tormented and ashamed artists at a time when homosexuality had not been invented. But sodomy was all the rage in a Florence bound by Catholicism. In 19 aggressively verbose scenes, several of which director Gregory Doran should have removed or cut, Sher lets the action wander discursively around, as if to paint an entire but irrelevant Florentine picture, with Stephen Noonan's suave Machiavelli, a grand masque, Leonardo's dreams of flying, threatening Savonarola acolytes and Philip Voss's gorgeously camp Gonfaloniere.

Charismatic Stephen Fagan makes a spectacular London debut as the too-sexually-enigmatic Vito. He equips himself with an Ulster accent rather than a rural Italian one, while exposing every handsome, muscular, naked bit of himself for minutes. Light's enthralled but appalled Michelangelo, who is inclined to rant, and Roger Allam's wily, worldly but not very passionate Leonardo, often accompanied by Simon Trinder's anachronistic gay boy, survey him in hopeless fascination. The artists' jealous rivalries over Vito, the anguish and selfloathing that the quarryman inspires in Michelangelo, and the celibate Leonardo's attempt at seduction, need developing as the play's main business.

The evening's triumph belongs to designer William Dudley, who creates a Stone Yard, complete with what resembles a huge, horizontal marble slab. When thrillingly raised to the vertical and transformed into a towering statue of David we witness a designer's imaginative triumph and a rare theatrical feat.

• Until 1 December (020 7722 9301).