“It wasn’t a great surprise. You know the hill of Montmartre is riddled with old gypsum mines.”

“So did you explore?”

“Oh yes. There’s a network of tunnels in there.” He nodded thoughtfully. So I rebuilt the privy with a shed beside it. The back of the shed slides open. The opening’s just behind it.”

“Did you tell anyone?”

“Not a soul, except you.”

Although the little garden wasn’t overlooked, Luc waited until darkness had fallen before he led Thomas out to the privy. He gave Thomas a covered lamp to carry. While Thomas waited, Luc entered the little shed beside it, and Thomas heard a wooden partition slide open.

“Bring the lamp,” Luc whispered. Thomas stepped into the shed and felt Luc’s hand guiding him through into the tunnel. “Turn left,” Luc whispered, “and walk twenty paces. Then you can uncover the lamp.” The surface under his feet felt stony.

When he uncovered the lamp, Thomas saw that he was in a high passage, about six feet wide, that led away into the distance. The walls were quite smooth and it was dry.

“Leave the lamp here,” Luc said. “Nobody can see the light from outside. We’ll go back for the girl now.”

She clearly had been in her early twenties. A fair-haired girl. She’d been hit in the face, but that hadn’t killed her. More likely the blow to the back of her head had done that. She must have fallen hard against something. Thomas wanted to ask, “How did it happen?” but he decided the less he knew the better.

There hadn’t been too much blood, and Luc had wrapped her tightly in several large tablecloths to keep it from spreading. The blood there was dry and black.

“You’ll have to get rid of the cloths. And the carpet might be stained,” Thomas said.

“I know,” said Luc. “If there’s a stain on the carpet, I’ll cut it out. Use the good bits. Burn the rest. No one will ever see.”

It was completely dark when they took the girl’s body out. They used straps to carry the corpse, which made it easier. It was a little tricky getting her into the shed and closing the door behind them, but they managed. Once they were in the passage, Luc closed up the entrance. After that, walking along the tunnel was relatively easy. When they reached the lamp, they put the body down. Thomas picked up the lamp and retraced their steps. He wanted to see if there were any drops of blood on the ground. He couldn’t find any.

“Where to now?” he asked.