What was that about impoverished regional theatres with no money, no audiences and no sense of ambition? In the period when most provincial playhouses shut up shop for the summer to save a bit of money, the Theatre Royal has opted to splurge on a large cast, full band and big set at the service of a melodious, venerable romp that has been resting on its laurels for far too long now.

A Funny Thing Happened on the Way to the Forum, Stephen Sondheim's first solo effort as composer and librettist, has always rather suffered by way of association. First you think of Frankie Howerd, then you think of Up Pompeii, then you think you must have seen it. True, in the course of its madcap plotting it manages to find a place for every cliche in the book, but at least Sondheim went direct to the book that they originally came from.

What is so surprising about A Funny Thing..., apart from the insouciant sophistication of Sondheim's deceptively simple songwriting, is the stamp it bears of its primary influence: Titus Maccius Plautus, the earliest extant Latin comic dramatist. You want to know where mother-in-law jokes and that old gag with the plank came from? You need look no further than Plautus himself.

Nor is it a one-man vehicle, as Frankie Howerd's triumph in the original West End production might have suggested. Martin Barrass is in top form in the taxing toga part of Pseudolus the Slave, and proves that there is more to the role than obsequiously following Frankie. But the real reason for the show's irrepressible buoyancy - and undoubtedly the reason it is attempted so infrequently - is that it demands a wide range of talent from a very substantial cast.

This is not a show you can skimp on, and thankfully director Damian Cruden goes for broke. True, the lengthy cast list offers a constellation of people you probably won't have heard of, but it just shows how unrelated concepts of ability and celebrity can be. Some of the cast, particularly the posse of gymnastic courtesans, are young guns on the way up; but it is the old blokes who make off with most of the honours.

Howard Attfield and Stuart Sherwin, as Senex and Hysterium, prove to be a hilarious pair of old pros in a show where the values of old-proism are paramount. Sherwin's biography informs us that he began his career in weekly rep. Does anyone remember what that was? They may do yet if shows like this continue to buck the trend.

• Until September 2. Box office: 01904 623568.