Any seasoned dance-goer knows that a wordy program note often signals an insubstantial ballet. Such was certainly the case with Gail Kachadurian's "The Cape," an empty-headed story ballet that had its Washington premiere last night at the Kennedy Center Opera House midway through yet another splendid performance by the Dance Theatre of Harlem. Kachadurian, a former member of the New York City Ballet and one of the first participants in the Dance Theatre of Harlem's Choreographers Workshop, claims to have based "The Cape" on a "particularly brutal historical incident," but we never find out just what it was. Set in no particular time or place -- neither the costumes nor the set of rocklike formations provides any clues -- the dance introduces a father (Bernard McClain) and son (Robert Garland) who seem to enjoy a close relationship. A Young Girl (Judy Tyrus), the son's main squeeze, enters, and the cape becomes a cutesy device that these three characters manipulate. Suddenly the father is arrested by a trio of menacing soldiers. He bequeaths the cape to his son, who proceeds, along with his sweetheart, to hide under it and to wear it as a skirt to disguise himself when pursued by the aforementioned soldiers. Unfortunately, these efforts come to naught. This blatant, deadly dull plot is communicated by means of the most hackneyed movement imaginable -- histrionic gestures, a distinctly unerotic love scene in which the duo roll together down a ramp, and much predictable swirling of the cape. The dancers imbue this vacuous material with their usual passion and precision. Only the choice of music -- Paul Hindemith's rousing, slightly Slavic "Symphonic Metamorphoses" -- proved inspired, and the Opera House Orchestra, conducted by Derrick Inouye, has rarely sounded so fine. The program began with a sleekly mysterious rendering of George Balanchine's "Serenade," with Lorraine Graves, Eddie J. Shellman and Lowell Smith turning in especially stirring performances. The entire ensemble shone brilliantly in Billy Wilson's "Concerto in F," a jazzy suite set to Gershwin's composition of the same name.