‘Yes. The dead man was probably lured there by this blonde girl who at an agreed moment abandoned him to his fate. At the finale, I’d guess, the killer entered and cut his throat, leaving the knife behind. They usually take the weapon away with them. This knife must have been significant, wouldn’t you say? I caught a glimpse of it. They picked it up with a handkerchief from the floor at the man’s feet. It looked foreign to me. And it wasn’t a zarin, which is the most popular knife in use in Paris.’

‘Zarin?’

‘It’s like a stiletto. The street gangs use it. For ripping and stabbing. Like this.’ She held an imaginary weapon in her hand using a backwards grip and demonstrated. Her face was impassive but her breathing was increasingly fast and shallow.

‘I’ve seen just that action somewhere,’ he said vaguely.

‘Well, this weapon was no zarin. It was short . . . fancy carved hilt.’

‘Ivory. Very distinctive. The dagger in this case was from Afghanistan,’ he said calmly. ‘A country in which Somerton had served some years ago, before the war.’ He calculated he was giving nothing away. It would be all over the newspapers tomorrow. And her response would tell him what he needed to know about Mademoiselle Raissac. Would she fall for the stimulus of the exotic blade he was offering and be inspired to spin out her story?

Yes, she would. Her eyes gleamed, her hands fluttered in expressive embroidery of her tale: ‘Well – there you have it, then! You should be looking for someone with a grudge going back to that time. A clear case of vengeance, wouldn’t you say? Someone with enough money and enough hatred, after all these years, to have the man very publicly killed. Payback for something murky in the past? That’s what the scene would have shown if your poor old friend hadn’t stumbled into the box prematurely. And now he’s got himself arrested and it all looks like an exhibition of jealous rage between two old codgers who ought to know better. A fit of rage that got out of hand.’

She considered for a moment and added: ‘They might not like that. An expensively staged act of retribution reduced to a sordid squabble. The customer who paid for his bit of theatre might not be entirely satisfied at the outcome.’

‘Not sure where you’re going with all this. He – whoever he is . . . Client of them perhaps? – can hardly say: “Excuse me – may we see that bit again, from the top?” can he? It’s not a dress rehearsal we’ve been treated to! More of a live – or rather death – performance.’

She scowled. ‘Nothing I can say will make you take this seriously. I’ve said enough. I’ve said too much. Who knows what he’s deciding at this moment? What they are planning? You’d better leave now. But before you go – I’ll remind you of our bargain. What was it? Six months in La Santé or spill my guts? Now, the question is, do I trust a policeman? (Am I naïve?) An English one? (Am I barmy?)’ She put her head on one side and considered. ‘No. I’m not stupid. And I’m not taken in by an affected lack of understanding that comes and goes, or by a handsome face and a pair of grey eyes that, with a little guidance, could find my soul. I’m going to take you for an honourable man. I couldn’t serve time in prison. Not even a day. I know what it’s like. The river . . . or the canal . . . would be my way out of that. So, unless you want my death on your conscience, you’ll keep your word.’

It was not the moment to tell her that so far she’d revealed nothing he could use. He . . . They . . . Nonsense. But sensing that she was still working her way through to offering something he stayed silent.

‘Look, can I ask you to do something for me before I give you the one bit of information I have that may help you? By coming here you’ve put me in danger. You must do what you can to put things right. No effort on your part involved! Agreed? Good.’

Ten minutes later Joe emerged into the sunshine. He looked around him, a man in an unfamiliar street, getting his bearings. He appeared oblivious of the passers-by though he was noting them through eyes narrowed against the sun: the two men strolling down the middle of the road, the tramp scavenging for cigarette butts in the gutter, the fashionably dressed young hostess on her way to her shift at one of the jazz clubs. Any one of them could be disguising an interest in a man leaving Mademoiselle Raissac’s apartment. Joe loitered on the doorstep as he’d promised Francine he would. She leaned briefly from the window, hitching up the shoulder of her silk gown, and called down to him: ‘Darling – I should have asked – can you make it two hours later next week?’

As a bonus, Joe made a show of adjusting his trousers with a louche smirk. Francine ducked back inside the room, unable to hold back a burst of throaty laughter. He looked at his watch, sighed with satisfaction and made off back to the square, whistling.

Better to compromise her good name rather than her neck, she’d judged. He’d been impressed by Mademoiselle Raissac. Mannequin? Dancer? No. Her modest stature might have deprived the boutiques or the Folies of her talents – but it was the stage of the Comédie Française that was the true loser. She would have graced any one of Molière’s plays. Dorine in Tartuffe? Perfect! What a performance the girl had put on for him! Emotion threatening to overflow at every verse end.

Charming girl, but she’d clearly been watching too many overblown dramas – onstage and backstage. Probably spent her spare time at the matinées in the Gaumont cinema, terrifying herself watching the adventures of Fantômas, Emperor of Evil. Joe shuddered as he recalled the image on the posters, known all over the world, but very particularly French in flavour: a mysterious gentleman, elegant in evening dress, inhuman green eyes glowing through his black mask, stalked a city-scape of Paris with giant strides. His left hand, kid-gloved, cupped his chin thoughtfully, as he selected his next victim. His right hand, slightly behind him, held in a backwards grip a blood-smeared dagger. And the grip was the very one Francine had demonstrated with such vigour. He wondered whether her storytelling was a part of her character, her way of enlivening an otherwise hardworking but humdrum life, or whether she was making a special effort to mislead the police.

The young nightclub hostess he’d marked down earlier must have doubled back. He was disconcerted to find her suddenly in front of him, coming towards him. How had she slipped by? He was getting careless. A few more strides and she was face to face with him on the narrow pavement. With an exclamation of apology Joe stepped to the side. But he chose to hop to the unexpected side, away from the road. Put out by his clumsiness, she dodged. They got in each other’s way, setting to the side and back again, partners in a country dance, disguising their impatience with embarrassed smiles. She began to speak to him. ‘I wonder if monsieur is looking for an encounter of a more intimate nature?’ she murmured, and then the familiar, shyly delivered, ‘Tu viens?’

Joe relaxed. A street walker after all. All was well. Training made him keep up his pretence of Englishman eminently satisfied by his experiences in Montmartre and he said politely: ‘Awfully sorry, my dear. Couldn’t possibly! I’m afraid you’ve picked just the wrong moment . . . if you understand me? Ha! Ha! Some other time?’ He rolled his eyes in an expression meant to convey both satiety and anticipation of a pleasure deferred and walked on.

Francine’s nervousness must be affecting him. ‘They don’t leave witnesses,’ she’d said, wide-eyed. Once round the corner, Joe’s pace increased. Belatedly catching her anxiety, he broke into a trot. Witness? He was thinking of another witness who’d had a clear view of the murder box. The chief witness, you might say, and one who had yet to make his full testimony. One he’d personally removed from the protection of police custody and left behind asleep in a hotel room. He began to run. Had he abandoned, unguarded, a loose end to be tidied up?