‘Everyone who enters gets one of these,’ explained Captain Warburton rapidly. ‘Also a notebook and pencil for copying clues. There will be six clues. You go on from one to the other like a Treasure Hunt, and the weapons are concealed in suspicious places. Here’s the first clue. A snapshot. Everyone starts with one of these.’

Poirot took the small print from him and studied it with a frown. Then he turned it upside down. He still looked puzzled. Warburton laughed.

‘Ingenious bit of trick photography, isn’t it?’ he said complacently. ‘Quite simple once you know what it is.’

Poirot, who did not know what it was, felt a mounting annoyance.

‘Some kind of barred window?’ he suggested.

‘Looks a bit like it, I admit. No, it’s a section of a tennis net.’

‘Ah.’ Poirot looked again at the snapshot. ‘Yes, it is as you say – quite obvious when you have been told what it is!’

‘So much depends on how you look at a thing,’ laughed Warburton.

‘That is a very profound truth.’

‘The second clue will be found in a box under the centre of the tennis net. In the box are this empty poison bottle – here, and a loose cork.’

‘Only, you see,’ said Mrs Oliver rapidly, ‘it’s a screw-topped bottle, so the cork is really the clue.’

‘I know, Madame, that you are always full of ingenuity, but I do not quite see –’

Mrs Oliver interrupted him.

‘Oh, but of course,’ she said, ‘there’s a story. Like in a magazine serial – a synopsis.’ She turned to Captain Warburton. ‘Have you got the leaflets?’

‘They’ve not come from the printers yet.’

‘But they promised!’

‘I know. I know. Everyone always promises. They’ll be ready this evening at six. I’m going in to fetch them in the car.’

‘Oh, good.’

Mrs Oliver gave a deep sigh and turned to Poirot.

‘Well, I’ll have to tell it you, then. Only I’m not very good at telling things. I mean if I write things, I get them perfectly clear, but if I talk, it always sounds the most frightful muddle; and that’s why I never discuss my plots with anyone. I’ve learnt not to, because if I do, they just look at me blankly and say “– er – yes, but – I don’t see what happened – and surely that can’t possibly make a book.” So damping. And not true, because when I write it, it does!’

Mrs Oliver paused for breath, and then went on: