Where did the five hundred pounds go?

A short mystery. The way to the answer runs through a puzzle.

Swipe.

The Envelope

Mrs Pike has worked behind a counter for thirty years, twelve of them on the river. She knows that the Thames is not the sea, that a boat is not a boat but a floating restaurant, and that tourists always ask the same question: when do we get to Hampton Court?

That morning Mr Hallam, who was travelling with a Welsh choir, set a brown envelope on the counter.

– Mrs Pike, dear. Keep this for me until lunch. My bag doesn't lock, and I'll be up on deck.

Inside were five hundred pounds in fifty-pound notes. Mrs Pike put it under the counter, behind the coffee tins, and pushed the jug back into place.

At half past eleven an order came in from the upper deck. Mrs Pike took it up herself and was gone for four minutes. She knows that for certain, because the radio played a song while she was away, and she knows that song is exactly four minutes long.

When she came back, the envelope was not behind the tins.

She searched twice, growing more anxious each time. Nothing. She sent word up to the deck. Mr Hallam was down within two minutes, and said nothing at all – just sat at the counter like a man who already knew.

– I'm sorry – said Mrs Pike. – I'll think it through first. Then we'll call someone.

Four people had been near the counter while she was upstairs.

Ron, the accordion player, who plays the same six songs every lunchtime, and who had been sitting at the end of the counter since eleven, by the socket, facing the serving hatch.

Marie, the cleaner, who was mopping in front of the counter, because somebody had spilled a lemonade.

Mr Hallam, who says he was on deck the whole time, though his coat was hanging by the counter.

The boy from the choir, who is eight years old, speaks no English, and had spent the morning touching, turning and pulling everything that wasn't set in concrete.

Mrs Pike didn't call anyone. She thought it through instead. The envelope hadn't vanished. Envelopes don't vanish. It had simply moved.

Fill in the sudoku

Tap an empty square first, then tap a number.

Nine squares in the grid are grey. The numbers in them turn into letters using the table below, read top to bottom, left to right across each row. As soon as a grey square gets the right number, its letter appears by itself in the nine empty boxes under the table. If no letter appears, the number is wrong.

The Envelope – continued

Mrs Pike waited until Ron had finished the usual six songs, then stood beside the instrument case.

– Ron. Open it.

He opened it. The envelope was lying beside the bellows, exactly where Mrs Pike expected it to be.

– Mrs Pike, I never –

– I know you never.

Through the serving hatch, Ron could see straight behind the counter. He saw the boy climb onto the cool box, and he saw what the boy took from behind the tins. He took it out of his hands, put it in the case, and was about to walk over to Mrs Pike – but just then someone called out that Hampton Court was coming up. Ron always plays the same tune when the palace comes into view, and by the time he had finished it, the envelope had gone out of his head.

An accordion player has a great many things in his head, but five hundred pounds isn't one of them. He'd never seen that much in one envelope before.

Mr Hallam got the envelope back and gave Mrs Pike twenty pounds. Mrs Pike gave it to Ron, and Ron played a seventh song for it.

The boy, meanwhile, was working on a button on Mr Hallam's coat.

A new mystery every week.