Road Show, Menier Chocolate Factory, London<br/>Singin' in the Rain, Festival Theatre, Chichester

Two brothers make their way from digging for pay dirt in Alaska to hobnobbing with the super-rich, in all-American style

Panning for gold is a cinch compared with getting a musical from first inkling to first night.

Stephen Sondheim has been sift-ing the rags-to-riches story of the real-life prospectors Wilson and Addison Mizner for 12 years, and the result is Road Show, being staged for the first time in Europe at the Menier Chocolate Factory, a favourite venue with the composer, who has been sitting in on previews.

Their father dead, the money gone, the Mizner brothers' true characters are exposed under the cold light of Alaska – as eager, pink-faced Addie painstakingly digs for gold, smooth-talking Willie, merely a gold-digger, slips camp and talks his way to instant wealth. By turns, the brothers hit and lose the jackpot with tireless ingenuity, dabbling in trade, sport, the stage, horses, the movies, marriage ... It is Addie's oversized collection of world souvenirs that inspires him to build a bigger house – and discover a talent for fantastical architecture. And so begins a property boom, to the rhythm of the Charleston, as the Roosevelts and Astors and Wanamakers vie for the most glamorous Xanadu.

To make a coherent whole of the many episodes of these volatile lives, collaborator, director and designer John Doyle contains the action between two banks of seating, the multitasking cast of 13 pinging to and fro like characters in a cartoon strip. And if the audience gets a little Wimbledon neck, it is compensated with lapfuls of dollar bills, as money flurries like tickertape over this vaudevillian parade. Key moments unfold on an ever-moving bedstead – death, love, journeys. Catherine Jayes directs the band and hearty ensemble singing, with Michael Jibson as Addie, David Bedella as Willie, and Jon Robyns as Addie's elegant, wealthy lover rattling through the numbers, among which, the love anthem "The Best Thing That Ever Has Happened" stands out. Road Show is a polished new sparkler in the Sondheim treasury, and maybe it's 18-carat, but it's certainly gold.

Plastic macs for the front row and a trough in the stage at Chichester Festival Theatre prepared us for a shower in Singin' in the Rain, but few could have expected the deluge that brings Act One to its spectacularly soggy end, Adam Cooper sploshing and galumphing with reckless glee. The title song and the bravura dance that Gene Kelly was said to have performed in one take in the original movie are the stuff of Hollywood legend. But here is Cooper, debonair as Astaire, racy as Kelly, "doing it in one take" every night. And, what's more, within moments of the show's second most demanding set piece, "Good Morning". That's classy.

Jonathan Church isn't holding back either, staging probably the greatest musical comedy film ever. Comden and Green's gags and set-up crank a little slowly for modern tastes, and we do not so easily ridicule an actress merely for having a Bronx accent, but, overall, the ultimate backstage musical holds up, 60 years on.

Designer Simon Higlett's Hollywood is sepia-tinted with an apricot blush, a fun factory where every gofer is a star-in-waiting, every errand an excuse to shimmy. With music from unseen players driving the action (musical director Robert Scott), Andrew Wright's sizzling choreography and a supporting cast on springs, the story unfolds of the crisis in silent movies that comes with the talkies (and here the sepia gives way to Technicolor, too). Cooper as matinée idol Don Lockwood, Daniel Crossley as cheeky chum Cosmo, and Scarlett Strallen as game girl Kathy Selden tap their way to a bright new dawn of elocution. Also on board are rubber-legged David Lucas as Don's diction coach, momentarily stealing the show in "Moses Supposes", and beguiling Ebony Molina, promising something a little hotter than wholesome Kathy has to offer in the dream sequence "Broadway Melody", while Katherine Kingsley's coarse Lina Lamont flounders in their wake.

Audiences have for centuries marvelled at the recreation on stage of something they would not look twice at outside, be it a horse and cart or a cloudburst. Knowing this, Church and company pull a giant stunt to close the show, and, not to spoil the surprise, suffice to say that the pakamacs come out again. I have seldom been so happy out of the sun.

'Road Show' (020-7378 1713 ) to 17 Sep; 'Singin' in the Rain' (01243 781312) to 10 Sep

Next Week:

Kate Bassett is drawn to Manchester by the mother of all performance art

Theatre Choice

The Pride, Alexi Kaye Campbell’s Olivier-winner about tangled relationships and shifting attitudes to homosexuality, runs at Sheffield’s Crucible to 16 July, with Daniel Evans. James Cordon is having a farcical blast in One Man, Two Guvnors: a 1960s update of Carlo Goldoni, complete with skiffle band, at the NT in London to 26 July.