Why should the National revive this evergreen mix of Plautine comedy and American vaudeville? One answer, as Edward Hall's joyously eclectic production proves, is that A Funny Thing Happened on the Way to the Forum is both a nostalgic throwback and a pioneering experiment - a musical farce predating The Producers by 40 years.
The book by Burt Shevelove and Larry Gelbart is a brilliant synthesis of stock characters from Plautus: the manipulative slave, the military braggart, the senescent lecher. But, into this cat's cradle of a plot about the attempts of Pseudolus to secure a bride for his master, Stephen Sondheim has inserted a group of songs that literally stop the show. Instead of advancing the story, they provide a musical respite; and, although Sondheim once said that "about three-quarters of the score is wrong", here a dozen wrongs add up to a right.
Hall instantly grasps the show's formal playfulness by making the opening number, Comedy Tonight, a comic climax: as Pseudolus itemises the evening's ingredients, including tumblers who pop out of a skip like jacks-in-the-box, they coalesce to form a high-kicking finale. Having started, as it were, at the end, Hall is then free to focus on the show's constituent parts. And nothing is more characteristic of Sondheim's elegant wit than a number like Free, in which Pseudolus hymns the joys of liberty, which include "the right to buy a slave for me".
But the production's success lies in its ability to draw together a whole range of performance traditions. Desmond Barrit, an RSC Falstaff, lends Pseudolus his own brand of roguish geniality: even the moment when his eyes lasciviously follow a courtesan's rotating hips is purged of offence by his unthreatening charm. Sam Kelly's hilariously goggle-eyed Senex, meanwhile, belongs to the old music-hall tradition of the hen-pecked husband.
Hall has shrewdly recruited the Right Size's Hamish McColl to play the quivering Hysterium. Philip Quast parodies the macho solemnity he has brought to earlier musicals by playing Miles Gloriosus as a vain sex object suspending his helmet from his private parts. The courtesans, dominated by Tiffany Graves's whip-cracking Gymnasia, come from some limitless beauty pool. Even the design, by Improbable's Julian Crouch, gives a Roman street an ironic antiquity. The result is a ministry of all the talents that does rich justice to this vertiginously funny show.
· Until November 2. Box office: 020-7452 3000.