The captain in charge of the military police squad was happy to let Friedrich go once they were away from the bar, and he headed back to Theresienwiese and Gisela. He was unnerved by what had happened: he should never have trusted Emil and told him his nickname; it just went to show you couldn’t trust a Pole. But at least it seemed no harm had been done. He didn’t even think the Americans had been listening properly to the accusations.

But one of the Americans had been listening. A young sergeant, he was surprised his captain hadn’t wanted to take the matter further. When they returned to their base later that day, he checked the extensive watch list containing the names of thousands of Nazis wanted for war crimes and other offences.

And there he found the following entry:

Das Frettchen: translates as the Ferret. Known in France as le furet and in Holland as de fret. Wanted for crimes against British agents and resistance fighters. PLEASE CONTACT MAJOR LEAN, F SECTION, SOE, LONDON WITHOUT DELAY.

Even before the war ended, it had become a matter of honour in the Special Operations Executive that every agent they’d sent into occupied Europe should be accounted for. The SOE demanded that justice should be sought for every one of the many who’d been killed, and that all agents who’d disappeared should be traced.

Major Charles Lean of F Section of the SOE – the non-Gaullist French section – had been haunted since December 1943 by Christine Butler’s murder. He’d recruited her personally, though at first she’d not seemed to be SOE agent material. She was in her mid-forties, relatively recently married and working as a secretary in the RAF. But a senior officer had discovered that her mother was French, and she was fluent in the language. All the background checks on her were excellent, so Major Lean arranged to meet her at St Ermin’s Hotel to see what he made of her, and by the end of their meeting he was highly impressed. She was a determined woman who felt under-used and unappreciated in the RAF and had a passionate desire to do what she could to help France. Despite her age, she was physically strong: she spent as much of her free time as she could hill-walking, and she sailed through her medical examinations.

Her husband, Nicholas, was a nervous man, medically exempt from conscription because of a breakdown a few years earlier, and it was agreed by all concerned that he shouldn’t know of his wife’s recruitment to the SOE. Christine Butler was given a few weeks’ training – Lean had to admit they were rushing them through these days – and sent to Dijon, where she was to link up with a British radio operator and sort out the Tractor circuit, which had been operating with very mixed results.

But the mission had not gone well from the start. The Captain – the man who ran the Tractor circuit – had insisted she be landed near Chaumont, which Lean felt was too far north of Dijon, and then they’d seemed to meander through Burgundy before reaching the city.