Plenty; Drama by David Hare. Directed by Mr. Hare.

The New York Shakespeare Festival has opened its indoor season with a handsome, intelligently acted production of a bleak and dispiriting play. In his ironically entitled ''Plenty,'' British dramatist David Hare is lamenting what he regards as the sorry state of England in the aftermath of World War II and the ''finest hour'' Winston Churchill described to his nation.

In the course of 12 scenes covering the period from 1943 to 1962, Mr. Hare attacks various manifestations of the welfare-cum-capitalist state and British establishment as represented by the Foreign Office. The symbolic center figure in ''Plenty'' is Susan Traherne (Kate Nelligan), who served as a British courier for the Free-French in World War II.

The episodic script traces Susan's mental deterioration as she fails to adjust from the exhiliarating highs of war-time dangers and commitment to the business-as-usual attitudes of a society that places pragmatism above ideals. After jobs with a shipping company and an advertising agency, Susan marries Raymond Brock (Edward Herrmann), an obliging but not exceptional diplomat with an aptitude for making money. Mr. Hare clearly intends Susan to be pitiable and even tragic. Instead, she developes into a tiresome and insensitive malcontent.

The Brocks and their stuffy but honorable senior colleague, Leonard Darwin (George Martin), are directly or indirectly affected by the 1956 Suez debacle, the event that accutely signaled Britain's imperial decline. Mr. Hare's handling of this and other post-World War II developments assumes a more than passing acquaintance with British and European history since 1945.

Outside of this context, ''Plenty'' functions mainly as a case history of a neurotic, increasingly hysterical woman. Always modishly gowned (by Jane Greenwood) and coifed, Miss Nelligan plays the role at a high pitch of emotional intensity. (She was named Best Actress of 1978 by the British critics for her performance of Susan at the National Theater.)

In her American stage debut Miss Nelligan is strongly supported by the actors cast as the men in Susan's life: Mr. Herrmann as the suavely upright and long-suffering diplomat whose career she destroys; Daniel Gerroll as the Cockney spiv with whom she hopes to conceive a child out of wedlock; and Kelsey Grammer as the fellow British volunteer with the Free-French who reappears for a squalid reunion in a Blackpool hotel.

The high level of the performance staged by Mr. Hare at the Public/Newman Theater is sustained by Ellen Parker in the utility role of Susan's loyal friend , Conrad Yama and Ginny Yang as a Burmese diplomatic couple, Madeleine Potter as a casually pregnant schoolgirl, Johann Carlo as a wan model, and Bill Moor as a subtly implacable Foreign Office personnel chief.

John Gunter's large-scale scenery threatens now and again to dwarf the action itself. The bold settings, beautifully lighted by Arden Fingerhut, feature an all-encompassing historic cyclorama, some vast canvases and large painted screens, and a rustic transformation scene that transports Susan back to the French countryside of 1944 and her own particular moment of hope and glory.