I plan," says Princess Lee Radziwill with a slight hint of defiance, "to go on with everything."

For the diminutive younger sister of Jacqueline Onassis that includes first nights, parties, couture fittings, cultural explorations, and a series of obliging male escorts; everything, that is, except her 15-year-old marriage to transplanted Polish nobleman Prince Stanislas Radziwill.

Last week, in London, the reserved prince filed for divorce from his pencil-thin, sloe-eyed princess. In fact, it was three years ago that Lee left "Stas" and their London home and moved back to the Fifth Avenue duplex she kept throughout her marriage. "I wanted to raise my children here," she says. "After all, I am an American, and I have always considered this my home."

By arrangement with Stas, son Anthony, 15, will remain in boarding school in England. Daughter Anna Christina, 14, stays with Lee in New York and Southampton. The real reasons behind the divorce remain curiously blurred. Some friends say the difference in their ages (Lee's 41, Stas 60) and interests (his are business and shooting, hers social and cultural) led to the breakup. (It is Stas' third marriage, Lee's second; her first was to publishing heir Michael Canfield.)

Others speculate that Lee's departure from Stas may be another in her feverish attempts to establish her own identity and to free herself from the long shadow cast by Jackie since childhood. After all, it was Lee, always considered by Jackie as "the prettier," who first sailed on Onassis' yacht and, in 1963, introduced Ari to Jackie. Since then Lee, with a little bit of help from her friends, has tried her hand at developing an artistic bent. Coached by Truman Capote, she tried stage (Philadelphia Story) and television (Laura)—both greeted by critics and public with polite reserve.

Recently, she has taped three segments of a proposed CBS television series—with old friend John Kenneth Galbraith, Gloria Steinem and designer Halston. Teaming up with Jackie, she has written a reminiscence of a childhood trip to Europe, called One Special Summer. On her own, she is writing a memoir of her early years for a reported advance of $250,000.

But, known for her extravagant spending, the lovely Lee would need an Onassis, or at least a Niarchos, to compete with Jackie's larger-than-life image as international first lady. Looking back on her marriage, Lee denies that any one romance was the cause of its demise. "It was stale for a long time," she said in her whispery voice. "It just took time to work out the details."