“You mean that Robert did the holdup business,” he said, “and there was a bit of a struggle, and the revolver went off, and then Mark lost his head and bolted? That sort of idea?”

“Exactly.”

“Well, that seems all right.” He turned to Antony. “There’s nothing wrong with that, is there? It’s the most natural explanation to anyone who knows Mark.”

Antony pulled at his pipe.

“I suppose it is,” he said slowly. “But there’s one thing that worries me rather.”

“What’s that?” Bill and Cayley asked the question simultaneously.

“The key.”

“The key?” said Bill.