He explained it to her, later, when they were naked in his bed. She was too afraid to take him to her place, but Frank had a car, and she figured that if X was watching her, he would find it more difficult to track her if she was in a stranger’s vehicle, moving at speed. So, she took a risk, the first one she’d taken in years, and the risk felt, at last, like it was worth it.

‘It’s to do with oxygen and carbon dioxide levels in your body,’ Frank said, stroking the soft skin around her nipples. They hardened in response. ‘When you freak out, you breathe in too much, take too much oxygen in. You need carbon dioxide in your brain too.’

Laura said nothing, just lay in the crook of his arm, letting his fingers move down, tracing the line from navel to groin, and marvelling at how unafraid she felt lying next to him.

‘You’re probably wondering how I know all this,’ he continued, but she didn’t take the bait. She didn’t want to know, in that moment. All she wanted was to lie there and enjoy something, for once. His fingers moved inside of her, and Laura, mesmerised by the sensation, felt like she were slowly waking up from a terrible nightmare. Which was exactly what she was doing.

Later, it transpired that Frank had been in a car accident, as a child. He’d struggled with anxiety ever since. Laura took this information, and thought about swapping it for her own story, the story of Bobby, and the letters, and X, and the codes, and her missing back molar. But she didn’t.

Even when they married, she didn’t.

Even when, red-faced and roaring, she pushed out her beautiful baby boy, and called him Robert, and marvelled at his white-blonde hair. Still, she said nothing.

She kept her secrets, and time flew past like water tumbling across a great fall, and her family took root, flourished, blossomed, and she smiled again.

And no letters came.

As she thrived, conversely, others did not, as if the scales of life found themselves unbalanced, and compensated accordingly. Mr and Mrs. Eveleigh divorced, grew older and developed health problems, and eventually, Bobby’s mother died, having never found out what happened to her boy. The newspapers loved the tragedy inherent in her not knowing, and, circling around her more insistently as she deteriorated, pestered her for interviews. She gave one, eventually, to a national tabloid six months before she passed. It was the usual fare: a harrowing plea for information that felt, because it was, like a drowning woman clutching at air. ‘The worst thing,’ the article concluded, ‘is not knowing what happened to Bobby. It’s the not knowing.’

Laura, the keeper of secrets, wondered how true this actually was. She suspected that knowledge would be just as dreadful for Bobby’s parents to cope with as the lack of closure. She suspected that not knowing had saved them some considerable distress. Who was to say? Mrs. Eveleigh had dwelt in anger for such a long time. It was unlikely any news in either direction would have revived her lost equilibrium. Her picture haunted Laura for a whole day as it stared out of the folded newspaper on the breakfast table. Mrs. Eveleigh’s eyes, rendered dark and flat by the poor print quality of the paper, followed her around the room, and, it felt, beyond. Why didn’t you do more? They said, accusingly. Why didn’t you tell us what you knew?