Peter Kroger and his wife Helen Kroger lived in New York where they were involved in spying for the Soviet Union. After the Second World War they moved to New Zealand. Later they settled in England and opened a shop selling second-hand books.

In 1955 Gordon Lonsdale, a KGB intelligence office, moved to England where he worked as a company director. While living in England he established a spy ring that included the Krogers and Harry Houghton. Over the next few years obtained a great deal of information on nuclear submarines and the location of secret military bases.

In 1959 Michael Goleniewski, a Polish intelligence officer, told the CIA that two Soviet agents were operating in Britain. Goleniewski was also able to identify one of these agents as working at the Underwater Weapons Establishment in Portland. This information was passed on to MI5 and eventually came to the conclusion that this agent was Houghton. By following Houghton the intelligence officers were able to discover about the activities of Ethel Gee, Lonsdale and the Krogers.

Kroger and his fellow conspirators were arrested on 7th January 1961. Peter Kroger was found guilty of spying and was sentenced to twenty years in prison. He was released in 1969 in exchange for Gerald Brooke, a British lecturer who had been arrested by the Russians for distributing subversive pamphlets.