Finally it was Boston's chance to see what New York had raved about all last year - David Hare's ''Plenty.'' A modern British political drama - which usually means ''does not travel well'' - ''Plenty'' is actually one of the more accessible imports. Instead of pelting the audience with oblique historical and political references, Hare has created one of the richest and most demanding female roles in decades. A romantic and ideological heroine, Susan Traherne, is clearly a metaphor for Britain's postwar decline.
Over the course of 19 years - 12 scenes that flash forward and back - she not only ages from her youthful exuberance as a British agent in occupied France but declines into madness as her hopes for her own and her fellow countrymen's abilities to ''improve our world'' grow dim. It is a demanding role, upon which the play's success almost entirely rests. Unfortunately, Katharine Manning at the Huntington Theatre is not up to the task.
A cool and stately blonde, Miss Manning seems to lack the inner fire, intelligence, and turbulence the role demands. It is not a compelling portrait, and the play is diminished because of it. The effect is not furthered by director Edward Gilbert's pacing and tempo: Where the scene changes should build a momentum augmenting the decline into dissolution and madness, they remain instead stiff and separate from the play's intent.
As for the supporting cast, it is adequate, although Gary Reineke as a high-ranking diplomat is a clear standout. Yet despite the production's lack of urgency, ''Plenty'' (at the Huntington Theatre until March 18) remains a provocative play well worth seeing.