That empty brown cardboard box Ricardo Montalban carried around in the Yard last winter has finally reached the local screen. It is a pleasure to report that the celluloid facsimile, of the box does not seem empty, nor does Mr. Montalban look nearly as silly carrying it. He is, in fact, quite believable in the role of police lieutenant Morales of Barnstable, a detective looking for an unknown murderer.

The murderer is ultimately caught by Harvard's Department of Legal Medicine, which operates in a highly scientific and unspectacular manner. "Mystery Street" is unspectacular, too, but it does hold interest without any movie melodramatics. Perhaps this is due only to director John Sturges' limited budget; nevertheless, the result is pleasing.

One of the things that make Montalban convincing in the role of a detective is that he is so often wrong. He tags the wrong man as the murderer from the start, and is only through the persistence of Harvard's "Professor McAdoo" that Justice triumphs. The Professor is played by a movie actor named Bruce Bennett, who, believe it or not, looks and talks like any number of youngish local scholars.

Ricardo and the Professor are looking for the murderer of a young "B-girl," (whatever that is) whose skeleton is discovered in a sand dune on Cape Cod. Their efforts are complicated by Elsa Lancaster, who plays the girl's unscrupulous land-lady, a character reminiscent of a malefactory Mad-woman of Chaillot.

"Mystery Street" has that aura of authenticity so valuable in a semi-documentary. Anyone who saw last year's shooting and reshooting in the Yard will be surprised at how spontaneous the carefully staged scenes appear. It is the sense of actuality and spontaneity that makes "Mystery Street" a quietly absorbing film.