Could a miracle survive in the modern world? What would have happened to the Mother of God if subjected to the methods of a meticulously trained psychiatrist? And what would befall that psychiatrist if forced to face a thing beyond human understanding?

These are some of the questions posed by "Agnes of God," John Pielmeier's provocative play now getting a powerful production at the Eisenhower Theater. The play uses the case of a nun accused of strangling her newborn baby -- and the mother superior and court-appointed psychiatrist who do battle over her soul -- to dramatize the theological conflict of faith vs. fact, and to plumb the mysteries of spiritual belief.

In this production -- starring Elizabeth Ashley as Dr. Martha Livingstone, Mercedes McCambridge as Mother Miriam Ruth and Maryann Plunkett as Agnes -- the drama is compelling, and the ideas become flesh and blood.

It's a tautly acted piece of work, directed with scientific precision by Michael Lindsay-Hogg, that purposely muddles matters to give them a strange plausibility. (Though, indeed, the play's fiction has some basis in real events.) Dr. Livingstone, no dispassionate investigator, is actually an ex- Catholic whose younger sister died in a convent of untreated appendicitis.

It develops that Mother Miriam Ruth took her vows of chastity after unhappy marriage, widowhood and final estrangement from her children. And Agnes, who may or may not be the repository of a miracle, is a crazy young woman who has hallucinations about her dead mother, who in life tortured her horribly with smoldering cigarettes.

In Dr. Livingstone's office -- evoked simply by a curving stretch of wood -- these things fit together gradually like a jigsaw puzzle, and the pieces (including some shockers) are the stuff of a riveting whodunit.

The performances are all superb, with Ashley's clipped cadence and sudden moves suggesting someone who would risk all to satisfy an impulse. McCambridge, controlled, hiding hands in her habit, negotiates the stage like deliberation itself. And Plunkett knows every inch of Agnes, from childlike innocent to keeper of terrible secrets. AGNES OF GOD -- At the Kennedy Center Eisenhower Theater through January 7.