But on one visit to the park something was not right. The park looked fine from its perimeter, but there was no chalk mark on the first two benches she checked, and as she approached the third one, she caught a glimpse of two men looking towards her from behind the bushes. Beyond the bench a couple were embracing in a most unconvincing manner, and past them, at the entrance to the park, she could make out three black cars parked together.

It was a trap, and she realised it was one the Captain must have led her into. She thought of his unscheduled visit earlier that morning.

What time will you go to the park?

What route will you take?

Her only possible means of escape was across the lawn into the wooded area, where she might be able to lose them. For a brief moment she thought of her husband, Nicholas: she had been forbidden to tell him about the mission, and he’d seemed hurt when she’d said she was going away somewhere but he wasn’t to worry. She was sure he thought she was having an affair.

She turned onto the wet grass but hadn’t taken more than a step or two when she was aware of being surrounded, a dozen men encircling her, none of them saying a word as her hands were pinned behind her back and something reeking of stale sweat was placed over her head.

When the hood was removed about an hour later, Christine Butler was in a brightly lit windowless room. She assumed it was in the headquarters of the Gestapo in Dijon, on rue du Docteur Chaussier, which happened to be near the cathedral.

She was tied to a metal chair, the straps around her ankles cutting into her skin. Her wrists were attached to the chair by handcuffs. The man sitting opposite her seemed out of breath.

What is your name?

‘Thérèse Dufour.’

Where are you from?

What do you do?

How did you get here?

His French was poor, and he didn’t follow up any of her answers.

‘You have my handbag: you’ll find all my papers are in order,’ she told him.

Finally he stood up, and she realised quite how overweight he was. ‘Never mind: your interrogation will start soon. You’ll soon have the pleasure of making the acquaintance of my friend das Frettchen!’ He laughed loudly, and was still doing so as he left the room.

She was left on her own, still strapped to the chair. When a gendarme came in to check on her, she asked him who das Frettchen was.

‘He means the interrogator: le furet.’ He bent down beside her, his mouth so close to her ear his moustache brushed against it. ‘Le furet has a terrible reputation. Don’t resist him.’

When she was once again alone in her cell, she remembered what le furet meant.

The ferret.

It was a few hours before the Ferret arrived. In that time she’d imagined a man who resembled one, perhaps with a long neck or a pointed nose, maybe beady eyes. She preferred that to an assiduous hunter and an efficient killer.