Stephen Sondheim's Pacific Overtures strikes me as the most original musical written in the past 40 years. Sure, I have cavils about this show, and about the new Donmar Warehouse production. But Pacific Overtures itself keeps wrong-footing me, happily so, and the intimacy of the Donmar space makes it intensely absorbing. It is the most surprising of Sondheim's shows, and the largest in spirit.

In this 1976 musical, Sondheim and the writer John Weidman re-tell the tale of the arrival of the Americans in Japan in 1853 from the Japanese point of view, and in a strong approximation of the devices, even the soundworld, of Japanese theatre. Not only are the female roles played by men, as in Kabuki or Noh theatre, but even the occidental roles are played as if by orientals. Of course, the words are English, and the music is essentially in the American/western tradition, so there are endless ironies about which point of view we are really receiving. In the Donmar staging - a co-production with Chicago Shakespeare Theater - the matter is further complicated by the rich ethnic mix of the cast.

The production is highly economical. Spectacular effects are not the point. Gary Griffin directs; Daniel Ostling designs. As in Kabuki, the stage is wooden, rectangular, ledged, and approached by a wooden corridor. The cast is dressed in black: wigs, asks, puppets, robes are used as in various Japanese forms. All parts are played by 10 actors. And straightway we're caught by the poetic pre-modern Nippon world created by Sondheim's music and lyrics.

At times Sondheim's wiseguy tendencies prove irksome. I don't adore the mannerisms of the average Sondheim vocal line, nor the occasional campy knowingness that cramps the spirit. I find a few of the performances here too campy also; and none of the singing is remarkable. But these problems do no predominate. Pacific Overtures is a deeply poetic piece of theatre. Its originality occurs at every level: in the orchestration, in the rhyme patterns, in the haiku-like lyrics, in the multilayered sense of humanity, in the irony that enriches, in the ambiguity that multiplies. Line by line, what is beign ignored and what is beign recognised grow side by side in your head like twin volcanoes. A deep poignancy emerges, and a great tenderness.