She’d not been there long when the message came through from Hervé, the radio operator: the word ‘thunder’ used three times, which meant everything had gone wrong. As far as they could gather, Christine had been arrested by the Gestapo and taken to their headquarters in rue du Docteur Chaussier. After that, communications ceased and it seemed Hervé had either been caught or killed.

A few days later, a report came through from another agent who’d been sent to Dijon from Lyons to find out what had happened. According to this, Christine Butler had undergone a brutal interrogation and torture at rue du Docteur Chaussier at the hands of a young Gestapo officer from Paris. It seemed that this officer had raped her so violently that she was sent to the infirmary at Dijon prison. According to a friend of someone who knew someone who worked in the infirmary, her internal injuries were probably fatal, but before her condition declined to that point, the same Gestapo officer who’d raped her turned up at the prison and had her carried out to the pavement, where he shot her dead.

According to the report, the agent was known by his nickname: das Frettchen.

The Ferret.

It fell to Major Lean to travel to the tiny terraced house in south London to inform Christine Butler’s devastated husband of her fate. For the first time in many years, the poor man had found a degree of happiness, and now all that was being taken away from him. Lean told him that his wife had been working for the RAF in Scotland and had been killed in an air accident. Sadly, no remains had been found. When he asked if there was anyone he could let know – maybe family or friends who could come round – Nicholas Butler shook his head and said there was no one. No family, no friends.

Major Lean added the Ferret’s details to all the watch lists, including an internal SOE one, and wasn’t terribly surprised – though he was horrified – when N Section contacted him the following May to say that a man matching that description, and with the same nickname in Dutch, appeared to be responsible for the death of an SOE agent called Peter Dean in Enschede, along with a woman called Frieda Mooren who ran the local resistance group.

Later, he also learned of other atrocities committed by this young, rather presentable Gestapo officer, possibly with an Austrian accent, who answered to the nickname ‘the Ferret’.

The SOE agreed that Major Lean would be the officer responsible for tracking him down.

In August 1945, Charles Lean was enjoying his first holiday in nearly four years when the receptionist at the hotel in north Devon handed him a message. A Christopher Stephens had called: please could he call him back on this number? Apparently it was urgent.

By the time Lean had incurred the considerable wrath of his wife and returned to London, Captain Stephens was ready to brief him. A man matching the Ferret’s description had been briefly detained by American military police officers after a bar brawl in Munich. There’d been some kind of argument and insults had been exchanged with a group of men, one of whom told the policemen that the man was in fact a Gestapo officer known as das Frettchen.