‘Now blow me down flat,’ Alleyn exulted, ‘if this isn’t the answer to the good little investigating officer’s prayer. See here, Fox, isn’t this a piece of the sort of scar we would expect to find? And look here.’

Very gingerly he turned the trout over and discovered, clinging to the other flank, a further rag of skin with the apex of a sharp triangular gap in it.

‘Sink me if I don’t have a look,’ Alleyn muttered.

Under Oliphant’s enchanted gaze, he opened his case, took from it a flat enamel dish, which he laid on the bottom step of the stile, and a small glass jar with a screw-on lid. Using his tweezers he spread out the piece of skin with the triangular gap on the plate. From the glass jar he took the piece of skin that had been found on the sharp stone under the Old ’Un. Muttering and whistling under his breath, and with a delicate dexterity, he laid the second fragment beside the first, opened it out and pushed and fiddled the one into the other as if they were pieces of a jigsaw puzzle. They fitted exactly.

‘And that,’ Alleyn said, ‘is why Mrs Twitchett met us last night smelling of fresh fish when she should have been stinking of liver. O Fate! O Nemesis! O Something or Another!’ he apostrophized. ‘Thy hand is here!’ And in answer to Oliphant’s glassy stare, he added: ‘You’ve done damned handily, Sergeant, to pick this up so quickly. Now, listen and I’ll explain.’

The explanation was detailed and exhaustive. Alleyn ended it with an account of the passage he had read in Colonel Cartarette’s book. ‘We’ll send out a signal to some piscatorial pundit,’ he said, ‘and get a check. But if the Colonel was right, and he seems to have been a conscientious knowledgeable chap, our two trout cannot exhibit identical scales. The Colonel’s killer, and only his killer, can have handled both fish. We do a round-up of garments, my hearties, and hope for returns.’

Sergeant Oliphant cleared his throat and with an air of modest achievement stooped behind a briar bush. ‘There’s one other matter, sir,’ he said. ‘I found this at the bottom of the hill in a bit of underbrush.’

He straightened up. In his hand was an arrow. ‘It appears,’ he said, ‘to have blood on it.’

‘Does it, indeed?’ Alleyn said, and took it. ‘All right, Oliphant. Damn’ good show. We’re getting on very prettily. And if,’ he summarized for the benefit of the gratified and anxious Oliphant, ‘if it all tallies up as I believe it must then the pattern will indeed begin to emerge, won’t it, Fox?’

‘I hope so, Mr Alleyn,’ Fox rejoined cheerfully.