The Donmar's new Pacific Overtures is a rather leaky vessel. It's only Sondheim's score that keeps it afloat.

In 1853, Commodore Matthew Perry went to open trade negotiations with the isolationist empire of Japan, taking with him a letter from the US president and a flotilla of fully armed battleships - because nothing haggles like a big gun. By 1976, Japanese corporations were buying up most of America, and Stephen Sondheim and John Weidman wrote Pacific Overtures, a musical about the modernising of a country nobody understood. It contrived - and contrivance is the key word here - to be by turns patronising, nostalgic, artifical, funny and, ultimately, paranoid.

Sondheim and Weidman wanted to see through Japanese eyes. They dutifully presented the visiting foreigners as clowns, carpetbaggers and barbarians, but in the process couldn't stop themselves portraying Japan as beautifully lacquered Pandora's box that someone should have had the sense to keep a lid on. In the best colonialist manner, they appropriated the musical and dramatic styles of kabuki theatre to tell their story, and Hal Prince's original Broadway production apparently sank under the resulting weight of costume changes, movable screens and conventions the audience didn't understand - such as the one where anybody in black is onstage to move things and is considered invisible. absolutely everybody is dressed in black for Gary Griffin's spartan 10-man revival at the Donmar Warehouse. It does not render them invisible, but neither can it save them from incoherence.

Kabuki is one of the most extravagant and stylised of all traditions, and keeping some elements while discarding others creates a peculiar form of theatrical pidgin. Griffin's actors haphazardly drop in and out of a knees-bent, head-bowed shuffle that is obviously meant to be authentically Oriental, but only makes them look like inexplicably separated halves of a pantomime horse. When they speak, they distort random syllables, stretching them into the long, guttural sighs of a man who has only just made it to the lavatory or squeezing them up into a series of herniated squawks. When a bunch of them are at it simultaneously, they sound like a symposium of yodas, and although this is meant to be alienating, we're only talking Japan here, not a galaxy far, far away.

Everybody takes on multiple roles, and, as in kabuki, all the female parts are played by men, with wildly varying degrees of femininity. Most impressive is Mo Zainal, whether as the demure wife of a samurai, a panicking girl serenaded by British sailors or the crow-voiced bimbo who snagged herself a shogun. Jerome Pradon is more obvious, and hilarious, as the shogun's mum, a cross between Maureen Lipman, Lily Savage and Lucrezia Borgia, poisoning her own son on the brilliant premise that if the Americans have nobody to deliver their letter to, they will simply go away.

The job of guiding us through these oddities of history and culture falls to the wry, commanding presence of Joseph Anthony Foronda as the Reciter, but the drama of Japan's westernisation is supposed to be played out largely through two characters: the samurai Kayama, who rises through government as he becomes less Japanese, and the fisherman Manjiro, who has spent time overseas. He begins the play convinced the battleships' arrival is a blessing, but gradually turns into a militant nationalist. There is a beautiful song where the pair take turns to recite haiku-like love poems, the samurai to his wife and the fisherman to America. The trouble is that only Kevin Gudahl's Kayama remains sympathetic into the second act, where he has a song that reveals his growing infatuation with the West. Richard Henders's Manjiro has to crouch in silence, making tea, and turns into a cipher. At this point, it becomes obvious what a disproportionate amount of the work - creating characters, advancing plot, even imparting historical information - is being done by the songs.

Sondheim has never been a big fan of the arts that conceals art. Where Mozart knocked off some of his loveliest tunes between visits to the billiard table, you can imagine Sondheim humming to himself as he tackles a crossword puzzle, jotting down clues he can use as lyrics. It is ironic that one of his most technically demanding musicals should contain some of his most heartfelt and straightforward words, with rhymes that are at times almost childlike. But their simplicity never stops them being witty, angry or poetic, and they are performed by all the cast with brio and conviction. Weidman's script may grind between cultures like the gear change on a 30-year-old Subaru, but Sondheim's score is smoothly universal.