"The Shadow Box," which opened last night at the Morosco Theater, is by far the finest play of the New York season, a beautifully realized drama of sensitive perceptions often as funny as it is moving. And the cast for Michael Cristofer's absorbing work is splendid under Gordon Davidson's finelytuned direction.

In one cottage on the grounds of a large California hospital a mother and her 14-year-old son have arrived from New Jersey for a reunion with Joe, his father. Maggie is consumed with the trivia of housewifery. She "made a ham," and she presumes that the little things Joe loves are unobtainable in California; and you get the feeling that for Joe; his time without Maggie must be a vacation.

In Cottage Two, bearded Brian is nimble with words and literary allusions. A visiting young man is out of sorts when they are joined by Beverly, Brian's divorced wife, a glittery type who delights inshowing the jewelry she's received from recent lovers.

In Cottage Three an old lady in a wheelchair chats with a doctor. Looked after by her plain, inhibited daughter, Agnes, old Felicity is constantly referring to Clare, her younger and obviously preferred daughter.

We will learn that the cabin occupants are dying and fated to stay where they are for the indeterminate period before their deaths. Critofer's theme subjugates the dying to what their deaths mean to those who have loved them.

Maggie has been unable to tell young Steve about his father. Her inability to face what Joe cries aloud for her to accept has an emotional truth of wrenching power. One can almost see Maggie's future years.

Brian's companion, Mark, is his lover, a hustler initially dazzled by Brian's flow of words and ideas. Mark sees that Beverly's presence will only upset Brian to pain wrenching phases of his disease. The avoided, then explosive, conflict of two loves is pushed to a dynamic pitch; Brian's wife and his lover can state their mutual awareness that Brian is not as brilliant as he seems, perhaps is even a fraud. As footless as Brian, Beverly will leave, and Mark will stand by the man who so curiously enriched his life.

In Cottage three, Agnes has concealed from her mother that Clare, the preferred sister, is long dead, that it is she who has been writing the letters from glamorous places which mean so much to Felicity. Since the facts about Clare could deprive her mother of the will to live, will Agnes continue to withhold them?

Incidents of the three unconnected histories weave in and out, a complexity of narration handled with imaginative theatricality. The dying speak to an unseen questioneer who might be an analyst or, perhaps, God. In Ming Cho Lee's expansive setting, one cabin is made to serve all three groups.

It should be noted that Davidson introduced Cristofer's play at his Mark Taper Forum, Los Angeles, restaged it for New Haven's Long Warf, with most of its cast now on 45th street.Once again, regional theater has enriched New York.