In the first weeks of the war, soon after she had joined MI5, Rosamund Kemp had done a stint of weapons training at an army range in Surrey. She had been quite proficient, particularly with a rifle.

But weapons training in the controlled environment of a firing range was very different from the real thing. And since coming to the Old Rectory at Harkham in the summer of 1940, there had been no further instruction or practice.

Now her hands were shaking so much she could hardly hold the revolver. It was a .38 Enfield, very heavy at one and a half pounds, with a difficult trigger requiring a full double-action pull for every shot.

She was well aware that an enemy agent was on the loose looking for the German girl. The fact that Tom Wilde had removed her did not necessarily mean the house was secure.

Rosamund sometimes heard more than her masters thought she did.

In this way, she had got the gist of the disagreement between Tom Wilde and Bodie Cashbone. The latter had a loud voice, and she wasn’t deaf. Rosamund wasn’t at all sure that she disagreed with Wilde’s action in removing the child.

An emergency telephone sat on her bedside table. But she did not want to risk making a sound, not with the intruder already in the house. Not even the soft sound of dialling or the whisper of a plea for help.

The footfalls had ascended the staircase and were now on the landing, not quite deadened by the carpet. She could hear her own breathing and feared that the intruder could, too.

She was not usually here alone. Most of the time there were agents here – highly trained agents who would know how to deal with a deadly situation like this. She tried to think. What would the intruder do when he turned her handle and found the door locked?

She heard the footsteps come closer, and the sound of the door handle turning. The door was locked and bolted, but the proximity of the intruder, just feet away from her on the other side, was terrifying. The door was made of of pine and not especially sturdy. She edged closer, held the pistol out at chest height, jerked the trigger hard and fired. The recoil jolted her backwards, but she kept pulling the trigger, again and again – three shots: left, centre, then right. She heard a groan and a dull thud.

A body falling to the floor.

Mrs Kemp waited, not sure what she was most afraid of: that she had killed him, or that she hadn’t. She had always known that MI5 would send potentially dangerous men here, but no one had told her she would have to use the gun she had been given.

Her arm was shaking. Her whole upper body ached.