‘That was after questions had been asked in the House about leakage of information to the enemy,’ said Ursula. ‘She took that very much to heart, Douglas, you know she did. And then that ship was torpedoed off the North Island. She was terribly upset.’

‘Personally,’ said Fabian, ‘I found her caution much more alarming than her curiosity. You’d have thought we had the Secret Death Ray of fiction on the stocks. She papered the walls with cautionary posters. Go on, Douglas.’

‘It was about three weeks before she was killed that it happened,’ said Douglas. ‘And if you don’t find a parallel between my experience and Ursy’s, I shall be very much surprised. Fabian and I had worked late on a certain improvement to a crucial part of our gadget, a safety device, let us call it.’

‘Why not,’ said Fabian, ‘since it is one?’

‘I absolutely fail to understand your attitude, Fabian, and I’m sure Mr Alleyn does. Your bloody English facetiousness—’

‘All right. You’re perfectly right, old thing, only it’s just that all these portentous hints seem to me to be so many fancy touches. You know as well as I do that the idea of a sort of aerial magnetic mine must have occurred to countless schoolboys. The only thing that could possibly be of use to the most sanguinary dirty dog would be either the drawings or the dummy model.’

‘Exactly!’ Douglas shouted, and then immediately lowered his voice. ‘The drawings and the model.’

‘And it’s all right about Markins. He’s spending the evening with the Johns family.’

‘So he says,’ Douglas retorted. ‘Well, now, sir, on this night, three weeks before Aunt Floss was killed, I was worrying about the alteration in the safety device—’

His story did bear a curious resemblance to Ursula’s.

On this particular evening, at about nine o’clock, Douglas and Fabian stood outside their workroom door, having locked it up for the night. They were excited by the proposed alteration to the safety device which Fabian now thought could be improved still further. ‘We’d talked ourselves silly and decided to chuck it up for the night,’ said Douglas. He usually kept the keys of the workroom door and safe but on this occasion each of them said that he might feel inclined to return to the calculations later on that night. It was agreed that Douglas should leave the keys in a box on his dressing-table where Fabian, if he so desired, could get them without disturbing him. It was at this point that they noticed Markins, who had come quietly along the passage from the back stairs. He asked them if they knew where Mrs Rubrick was as a long-distance call had come through for her. He almost certainly overheard the arrangement about the keys. ‘And, by God,’ said Douglas, ‘he tried to make use of it.’