Captain Hector Boyes, British Naval Attaché in Oslo, receives an anonymous letter offering German technical secrets. He is requested to signal interest by changing the BBC World Service's German broadcast announcement to "Hullo, hier ist London". He arranges this and a week later receives a parcel with a 7-page typewritten report (which becomes known as the "Oslo Report") and components of a prototype proximity fuse. They come from physicist Hans Mayer, director of the Siemens communications laboratory, who hopes to weaken the Nazi regime by revealing details of military secrets. Mayer arrived in Oslo on October 30, on a business trip, and typed the letter and report on a typewriter borrowed from his hotel.
The Oslo Report is initially considered a fake by British intelligence but is ultimately accepted. Mayer is sent to a concentration camp in 1943 for criticising the Nazi regime but never suspected of spying. He survives the war.