‘It is a kind of insanity, in the sense that it is also a kind of dreaming awake, a link, if you will, between dreaming and insanity. It causes the sufferer to act on false impressions as if they are real.’

‘I see.’ You glance at the jurors. ‘For example, a woman might believe an infant to be hidden under a hedge and set about looking for it, though there is no such thing?’

‘Yes! Exactly that. In such a state, one of my patients wrote an entire symphony yet next morning remembered not a single note. Another walked the lanes around his own estate all night, shot a fox and dragged the body back all the way through his own top field. Next morning, he swore he’d been in bed the whole time and he’d still believe that, too, except the whole thing had been witnessed by the local priest.’

‘Those were somnambulistic trances? Sleepwalking, in colloquial terms?’

‘Correct.’

‘Could the same state be produced by an excessive consumption of opium?’

‘That’s what excited me about your case, Mr Pettigrew. I believe, quite strongly indeed from what you’ve told me, that what your client experienced was akin to a somnambulistic trance, except of course we’re talking not about a sleeping state but a soporific one. Under an excess of opium, as in a dream, a user might also act under false impressions. The delirium in this instance created by the drug itself. But the memory of those actions could then be obliterated by the stupefying effect, resulting in the same split in consciousness.’ He hooks his fingers on his own lapels, as if he’s the prosecutor, rolling out his argument like a tailor’s cloth. ‘These matters, gentlemen, are as exciting, as reliable, as any of the century’s scientific advances. You may have faith in that.’

You look pleased with yourself again, reaching up to finger your wig.

But frustration knots my guts. This might be a good lawyer’s trick, but it siphons up all my own doubts, all my own fears. As if my own defence will be the very thing that seals my guilt.

This is planting someone else’s idea like a cuckoo in my head. If you killed her, Frances, this is how. Twin shapes, shadows moving in the dark, until one goes cold. But is that real? I know that blackness all too well. My heart staggers behind my ribs. It would have been unknowing. Of all Lushing’s paid-for words, I cling to that.

When it’s his turn, Jessop tosses his papers onto the barristers’ table. ‘Insanity! That is indeed an accurate assessment of this entire defence.’

‘Well, no, that’s not what I –’

‘Do you mean to suggest that the prisoner could have killed Mr Benham, taken herself upstairs, killed his wife, cleaned the carpet and the knife, and then – presto! ‒ got into bed, all in this alleged somnambulistic state?’

‘Oh, yes, very possible and I should say –’

‘The savagery to butcher them both, yet the presence of mind for all that . . . tidying up.’ He huffs out a laugh. ‘It’s hardly likely, Doctor. Those are not involuntary actions. Not some pitiable automaton, but a person making decisions, acting with care and deliberation, even self-interest.’