We were out and rushing through the muddy fields in seconds. As we approached, we could see that it was indeed young Buttons, standing poised on the wooden footbridge over the Jesus Lock, staring down into swirling dark waters.

Was he Dillie’s killer, returning in guilt and horror to the site of his transgression? Perhaps to the exact spot where he had dumped her, still alive but unconscious, and where she had met her watery end.

The rush of the river and the hiss of the late summer downpour masked our footsteps, and the boy heard nothing as we approached. We came within ten feet and Holmes put his hand on my arm to stop me. With a finger to his lips, he shook his head.

As we watched, Buttons leaned over the railing, mesmerized by the churning current. Lightning flashed again and lit up the scene for a brief flickering second. A moment later came the boom of thunder.

We had not made a sound, but in that strange way that one feels the regard of another, the young man sensed our presence and turned. His face was ghastly pale, and his hair was dripping as it was the first time we had met him at Baker Street. He was not wearing his glasses, and his white face was drenched from the rain and, I could only assume, tears.

‘Mr Buttons,’ said Holmes calmly, ‘please step away from the railing.’

‘Dillie died here,’ said Buttons in a strange, high-pitched voice. ‘Last night.’

‘Yes, we know,’ said Holmes.

‘The river … the river …’ The boy turned and looked at the rushing waters. ‘It washed away her sins,’ he murmured.

‘How did she get into the river?’ asked Holmes.

The boy did not reply but kept his eyes on the water. Holmes slowly inched closer. I stayed back, fearing the boy would panic.

‘I have just been in your room, Mr Buttons. It is clear to me that Dillie visited you there last night,’ said Holmes.

Buttons looked up at my friend in alarm, then looked at me.

‘The signs were unmistakable,’ I said.

The boy looked from one of us to the other, and sensed he was lost, though not sure how. ‘She came to get me.’

‘Get you?’ Holmes asked.

‘It was such a surprise. I never thought. But she … but she …’ He closed his eyes.

‘Let us start at the beginning. What time was this?’ said Holmes.

The boy spoke, his eyes remaining closed. ‘Two o’clock or maybe three. She woke me. She had brought her valise, all her things. She asked for my help. I had promised to help her if she ever needed … she told me that she wanted to run away with me. Right then. With me.’ He gestured vaguely. I noticed that his right hand was wrapped in something white, with a dark stain on it.

‘Holmes, his hand!’ I whispered.