With conspicuous severity, Ken Loach has made a picture about exploited Mexican immigrant workers cleaning offices in downtown Los Angeles in his first film in the United States.

Adrien Brody plays Sam, the middle-class Anglo union organiser who incites them to action, and falls in love with one: Maya (Pilar Padilla) the younger sister of Rosa (Elpidia Carrillo) the careworn woman who has risked everything to get Maya across the Rio Grande.

The result is a thumpingly unsubtle movie, a Lehrstück, in which Loach appears insufficiently at home in his new American location to include any of his habitual light touches of humour and nuance.

In fact, it is almost as if Loach is consciously repudiating these bourgeois irrelevances, in the service of reminding us of the scandal of LA making itself wealthy by gouging the urban peasant class, and exploiting the wetbacks who do the dirty jobs.

This is megaphone cinema. But like a megaphone, it is intermittently capable of crude power and effectiveness. Loach highlights the grisly caste system which is a metonym of corporate prosperity: the immigrant office cleaners who are made to feel invisible, who do not show up on the white-collar feelgood radar.

Bread and Roses is often as two-dimensional as a placard, or a billboard. That is disappointing in so experienced a director. In his other movies, there has been no contradiction between subtlety and political commitment. But this cannot entirely cancel its sense of moral and political seriousness, and its powerful performances from Padilla and Carrillo.