KAGEMUSHA -- At the Avalon.

Underneath the trappings, "Kagemusha" ("The Double") is just another what-if story, without the fancy psychological embellishments one expects of an Akira Kurosawa film. But as it's a 16th-century Japanese costume epic with lots of battles, those trappings are nifty.

The what-if concerns a petty thief who has been saved from crucifixion in order to serve as a stand-in for the head of a feudal clan, whom he closely resembles. What if the lord were to die, advising his retinue to conceal his death from his enemies for three years, and the thief therefore were forced to play the master full-time?

Surely Kurosawa, the maker of "Rashomon" and "Seven Samurai," should be showing us the double slowly turning into the original or needing to destroy the dead master to free himself -- something deliciously complicated. But this thief remains a timid imposter. His rivalry of the master is represented by one silly nightmare, his attempt to grow into the part is only one quickly thwarted moment of bravado.

And after the pretense is over, the double's continued interest in the scene of which he has been part seems demented, because a mystic bond between the two has never been established. He has never been the leader, never been more than a dummy taking orders, and the pretentions to emotional identification ring false.

Little is done to make the premise plausible. Too many people are in on the deception for it to be possible for the secret to be kept in those spy-ridden times. The double is twice allowed to go free -- when he at first refuses the job, and again when his usefulness is over -- out of a magnanimity not otherwise apparent in these ruthless circles. And although the same actor, Tatsuya Nakadai, plays orignal and double, his performance as the frightened thief is so different from that of the swaggering chief that one cannot mistake the two.

The giggling inattentiveness of the late man's mistresses, his son's tiresome jealousies, and his brother's ironic instructions to the double, "Act natural -- like him," are therefore close to being comic.

The real interest in this very long film is its choreography. Japanese medievalism is picturesque and there may be a vogue for it because of "Shogun." But what Kurosawa has done in the fighting scenes would be pictorially amazing anyway, because of the way he moves crowd segments to create large-scale moving pictures. At its best, the film has the power of huge tapestries to represent the details and the force of the whole simultaneously.

Alternating with these scenes are some very bad ones -- the dream sequence which takes place in a painted desert; a symbolic rainbow that is garishly phony, a body-strewn battlefield on which a live horse moves eerily, until a dozen other horses and bloody soldiers carry the same movement to absurdity.

What an irony it is to see what is obviously the work of a master juxtaposed with these fakes.