The Shout have carved a niche in the world of choral-singing. Thanks to the leadership of composers Richard Chew and Orlando Gough, they have developed an energetic, improvisatory style, and sing without scores or conductors. The aim of their Tall Stories show is as ambitious as it is worthy: to dramatise the experience of early 20th century immigrants in New York. This gives the 16 singers the challenge of convincing not only as musicians but also as actors.

Yet the show is neither conventional theatre nor opera. Instead, director Rufus Norris attempts to create a hybrid. Described as a "staged song cycle", the numbers (by Gough and Chew) are knitted together into a simple narrative. We experience the hopes and fears of refugees as they travel to New York and try to build their lives there.

And build them they do: the finale of the first half depicts the construction of New York's skyscrapers. The cast clamber up the towers and windows of Katrina Lindsay's set, a metaphor for the aspirations and achievements of America's immigrants.

All 16 singers are used throughout, and the emphasis is inevitably on collective experience rather than specific characters. The strongest parts of the show are numbers such as Silence, a simple and powerful lament for the dark side of the American dream. Much less successful are the moments that highlight individual members of the cast. As actors, few are capable of a commanding presence. Instead, sections such as High Hopes, in which each singer introduces a new character, are dramatically unconvincing.

None of this would matter if the musical material were strong enough to sustain the narrative. However, many of the numbers rely on cliched riffs and progressions. An apparent diversity of vocal techniques - from jazzy scat to operatic melismas - becomes part of a bland musical soup.

Tall Stories tries to rethink musical theatre. But, ultimately, it is limited as drama, and unsatisfying as music.

· Ends tomorrow. Box office: 020-7223 2223.