BEFORE AGNES DE MILLE introduced a dreamy 20-minute ballet at the end of the first act of "Oklahoma!," Broadway musicals were mostly grin-and-shuffle affairs populated with tap dancers and leggy blondes sporting pretty smiles. Dance was meant purely to entertain and hardly had a thing to do with a show's storyline. But in 1943, de Mille, working with the now-legendary creative team of Richard Rodgers and Oscar Hammerstein II, set out to change that. The daughter of a playwright and niece of extravagant Hollywood movie director Cecil B. de Mille, in the 1930s and '40s, Agnes de Mille found her niche, if not her fortune, as a dancer and choreographer piecing together a career that spanned New York and Europe. De Mille's choreography -- for the concert stage, for traditional ballet companies and for musical theater -- proved a charming blend of ballet, modern and folk dance idioms that denoted a quintessentially American approach.

Set in the early 20th century, just before the midwestern territory joined the union, the musical is filled with cowboys and horses, a pioneering spirit and romantic intrigue. But unsettling undertones permeate as well: a sinister outsider and subtle hostilities that lie beneath the surface.

When "Oklahoma!" opens Thursday at Wolf Trap's Filene Center, the de Mille legacy will still be felt, although this production, based on British director Trevor Nunn's acclaimed 1998 Royal National Theatre revival, is choreographed by Ginger Thatcher. Thatcher danced de Mille works, among them de Mille's beloved cowboy ballet, "Rodeo," which more than 60 years ago whetted the appetites of the original "Oklahoma!" creative team for de Mille's pioneering style.

"She was a terrific, groundbreaking choreographer, in particular with 'Oklahoma!' " Thatcher said of de Mille. "Up until 'Oklahoma!' a dance number or a song didn't necessarily have anything to do with continuing the telling of the story. There wasn't a seamless theme of ongoing action and storytelling that continued through a musical. 'Oklahoma!' was really the first musical that incorporated that."

This revival strips away the sugarcoating, taking a more serious view of pioneering life in the Oklahoma Territory than the original production did. Thatcher modeled her choreography for the national touring production on Tony Award-winning choreographer Susan Stroman's work for the 1998 Royal National Theatre revival in London.

"Susan Stroman felt that she should take a new interpretation," Thatcher said. "Not radically different, but examining the dark side of what was happening in Oklahoma Territory at that time." This license to explore the historic social, political and racial undertones of the period is, Thatcher said, an expansion of what de Mille did in 1943: "It was really Agnes de Mille who paved the way for that."

Thatcher discovered ways throughout the musical to present the conflicts vividly through dance. In "The Farmer and the Cowman," the rivalry between ranchers and farmers is told explicitly through body postures and gestures, little off-to-the-side scuffles and competitive dancing that heightens the drama.

The biggest change from the de Mille version comes during the famous dream ballet. In the original production, the leads didn't dance for practical as much as for dramatic reasons: It proved near impossible in 1943 to find musical theater performers who could muster de Mille's challenging balletic sequences.

Stroman, and now Thatcher, have done away with the dream dancers. The lead performers -- Amanda Rose and Brandon Andrus -- fully dance the ballet. Though it took nearly a year to cast Rose as Laurey, the leading ingenue, Thatcher finds that uniting the star and the ballet sequence makes for a more integrated and heartfelt expression of Laurey's fears and hopes, which is really what "Oklahoma's!" creators were after from the outset. Rose, a well-rounded young actress who sings and dances, said, "If someone can do all three [sing, dance and act], it really furthers the plot. I find that dance is just another way of expression. In musicals, when things get too emotional, you just have to dance."

OKLAHOMA! -- Thursday through June 27. Wolf Trap's Filene Center, 1624 Trap Rd., Vienna. 703-218-6500.

Brandon Andrus (Curly) and Amanda Rose (Laurey) sing -- and also dance the "dream" ballet -- in this revival of "Oklahoma!"