David climbed the oak stairs to Dr Bowler’s rooms with his notes for this afternoon’s tutorial tucked under his arm. He marvelled at the fact that his mentor would have dry sherry and Bath Oliver biscuits waiting, while not half a mile away women were queueing for a few ounces of butter.

He had come up to read Mathematics at Oxford just days before war broke out and no doubt once he had received his degree next year he would be in the thick of things equally quickly.

‘Come in, Rokeby.’

Bowler was sitting, as usual, near the far side of the fireplace and David out of habit moved to take the opposite seat, but it was already occupied.

‘Adrian Fisher.’ The young man wore an impeccably cut pre-war suit and his hair unfashionably long. David braced himself for a limp handshake but the grip was reassuringly firm.

‘Fisher is from the…
‘Ministry of Supply.’ Fisher interrupted.

‘I’ll come straight to the point, Rokeby. I am recruiting. We need bright young chaps and girls.’ His gaze went across the room to where Gillian Bishop had just entered with a tray of tea from Bowler’s tiny kitchen. ‘You, Rokeby, and Miss Bishop are among the best in your year and thereby have come to our notice. We need top flight mathematicians to help us with some thorny problems.

When you return to your rooms both of you will find an envelope. In it is a Caesar Cypher. Solve the code and you will find it is a set of instructions. You will need to be sharp. You may be a natural, but these codes require a specific key, which I have given you. Write the key at the beginning and then the remaining letters of the alphabet in order to produce the substitution code. Good luck.

With that Fisher excused himself and left three thoughtful people drinking tea.

For two days David and Gillian struggled separately, then for another two days together. Exasperated, Gillian cried, ‘He said he had given us the key. But where is it?’ The familiar music playing quietly on the radio ended.

‘This is the BBC Home Service, and that was Grieg’s piano concerto in A flat minor.’

David looked at Gillian and she stared back.

‘B Sharp and A natural.’ They shouted together.

Ten minutes later they were reading their instructions.

‘Bletchley Park? What on Earth is there, I wonder?’