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She was born in Kasauli, India, and educated in Berkshire, studying Russian and French. After school, she worked in London before joining the Allied Control Commission and moving to West Germany. There she met her future husband, Ruari Chisholm, and began working for British Intelligence.
Her husband was a spy who headed the MI6 station in Moscow but worked under the guise of a visa officer. When Oleg Penkovsky offered Soviet military secrets to MI6, she became the go-between. To all outward appearances, Janet was an ordinary mother of three children. She would meet Penkovsky in a local park and he would offer what appeared to be sweets, but were actually disguised documents containing military secrets about the Soviet nuclear weapons arsenal.
Their cover was eventually blown by another British agent, George Blake, who had defected to the Soviet side. Penkovsky was quickly arrested and executed and the Chisholms were deported. Controversy surrounds Penkovsky's death, with many believing that MI6 put him in danger after Blake confessed all to the Soviet officials. For their part, MI6 have always maintained that he insisted on continuing to feed the British information even after his life was threatened.
The Chisholms went on to work in Singapore and South Africa until Ruari's retirement. He had planned to write a memoir but died of malaria before he could begin. Janet continued to travel around the world until well into her 70s. She refused to talk about her experiences in Russia and took all of the secrets she learned to the grave.