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Neatly dressed in his suit, Hans Ferdinand Mayer was every inch the unassuming corporate executive. So, when he asked to borrow a typewriter from his hotel in Oslo, nobody could have guessed he would use it for one of the most extraordinary intelligence leaks in history.

Mayer's gloved fingers punched out the details of Nazi Germany's most sensitive military operations and, when he had finished, he immediately dispatched his documents to the British — who did nothing.

Why did the British ignore Mayer? Did they fail to pick out a crucial signal amid the noise of detail — or was something else going on?

This episode of Cautionary Tales is based, with permission, on Tom Whipple’s book The Battle of the Beams, which is available from all good booksellers.

For a full list of sources, see the show notes at timharford.com.