In 1943 Jeannie Rousseau, as she was then known, was an interpreter in Paris for an association of French businessmen, representing their interests and helping them negotiate contracts with the German occupiers. She was young and attractive. She spoke flawless German. She was a favorite with the German officers, who were completely unaware that the woman they knew as Madeleine Chauffour had been reporting to a French intelligence network, the Druids, organized by the Resistance.

Getting wind of a secret weapons project, she made it her mission to be on hand when the topic was discussed by the Germans, coaxing information through charm and guile.

"I teased them, taunted them, looked at them wide-eyed, insisted that they must be mad when they spoke of the astounding new weapon that flew over vast distances, much faster than any airplane," she told The Washington Post in 1998. "I kept saying, 'What you are telling me cannot be true!' I must have said that 100 times."

One officer, eager to convince her, let her look at drawings of the rockets.

Most of what she heard was incomprehensible. But, blessed with a near-photographic memory, she repeated it in detail to her recruiter, Georges Lamarque, at a safe house on the Left Bank.

In London, intelligence analysts, led by Reginald V. Jones, marveled at the quality of the information they were receiving from Paris, notably a startling document called the Wachtel Report. Delivered in September 1943, it identified the German officer in charge of the rocket program, Col. Max Wachtel; gave precise details about operations at the testing plant in Peenemünde, on the Baltic coast in Pomerania; and showed planned launch locations along the coast from Brittany to the Netherlands.

Relying on this information, the British organized several bombing raids against the plant, which delayed development of the V-2 and spared untold thousands of lives in London.

In "1940-1944: The Secret History of the Atlantic Wall" (2003), the historian Rémy Desquesnes called the Wachtel Report a "masterpiece in the history of intelligence gathering." When Mr. Jones asked who had sent the report, he was told that the source was known only by the code name Amniarix, and that "she was one of the most remarkable young women of her generation."