‘He’s quite a tall man, the victim?’

‘About five feet ten inches and, though he must be in his mid-fifties, his musculature is in good condition.’

‘Difficult to subdue a man of that height unless you are yourself taller and more powerful, you’re thinking?’

‘As are you, Commander. He’s hardly likely to oblige by standing there, sticking out his chin and closing his eyes! A man like this would have fought back against a perceived assailant.’ The pathologist pointed to the hands and forearms. ‘No signs of wounds received during self-defence, you see. No attempt to repel a knifeman. It’s my theory that his attacker came up behind him while he was still seated, seized him – possibly left hand over his mouth – and slashed and sawed his throat from ear to ear. Standing behind your victim, you would not be showered by his blood which would be projected out and down. You then allow the body to flop forward on to the padded edge of the booth where the obliging upholstery absorbs most of the litres of blood. Velvet, quilted over cotton wadding, I understand.’

‘And if you’re a careful killer,’ added Joe, ‘and I’m sure our man was just that, you’d have taken off your opera cloak and put it on the peg by the door, murdered your victim and then put your concealing cloak on again before leaving. If you’ve timed it just right, your exit will coincide with the moment everyone was streaming out of the theatre.’ He knew that moment. Everyone preoccupied with his or her own immediate plans . . . taxis, supper, romance. No one wanted to catch the eye of a stranger in the crowd.

Joe went to fetch a chair, placed it at the foot of the marble slab and sat down on it. ‘Doctor, would you mime the action of the killer as you judge it to have been carried out? I’ll be the victim.’

‘Of course. And, in the pursuit of authenticity – a moment – I’ll just fetch the weapon.’

Moulin bustled away into his office, returning with a cardboard box filled with material conserved from the corpse. ‘We’re holding all this until the police and the magistrate are satisfied. You know the routine?’ He waited for Joe’s nod and went on: ‘The personal effects will be returned to the next of kin. Not sure they’ll want to keep this as a souvenir though,’ he said, producing from a paper bag a dagger with an eight-inch blade and a carved ivory hilt.

‘I’d like the lady to take a look, if she can bear it,’ Joe said. ‘Just in case she can identify it as her husband’s own property.’

‘You can take hold of it,’ said Moulin, offering it by the point of the blade. ‘It’s been tested for fingerprints and cleaned up. No prints, by the way. It had been wiped clean – just some unusable smears left.’

Joe took the object with distaste. ‘Afghan.’ He turned the blade flat and slid it over the back of his hand, slicing through a few hairs. ‘Sharp as a razor.’

‘It would need to be to go quickly through such an amount of muscle and gristle. The throat is not an easy option. But it is quick and sure. Think of pig-killing. In my village they always go for the throat. And a pig’s flesh has more or less the same density and resilience as a man’s. This knife went upstairs to the laboratory for inspection. Under the microscope you can see the signs of the use of a sharpening implement on the blade. Very recent sharpening was done. Perhaps with the killing in mind?’

‘Ah? A workmanlike tool. Not a cheap blade but not lavishly produced for display, I’d say. It’s not as ornate as many I’ve seen. An inch or so shorter than most. Discreet. An efficient killing blade.’

‘Indeed. Now this is what I think happened. For the record – I’m five foot eight inches tall, so we’re possibly looking for someone two to four inches taller. And almost certainly more powerfully built.’ The doctor took the knife in his right hand. He mimed taking off his cloak and hanging it up then he moved silently behind Joe who leaned slightly forward in the attitude of someone engrossed in the performance on the stage below.

‘Ah! In the dark and with your head tilted forward like that it’s not so easy to get a hand around your mouth. I’m going to change my plan slightly,’ said Moulin.

He grasped Joe by the hair and pulled his head back, applying the dagger blade to his exposed throat. Joe could not repress a shudder as the cold steel gently touched the skin behind his left ear.

‘Yes, that’s how it would have been done!’

‘What about the noise, doctor? Would he have had time to let out a scream?’

‘Oh yes. Think of any pig you’ve ever heard being slaughtered. They manage a few seconds of hideous squealing before their voice box is cut. It must have been done at a moment of intense surrounding noise.’

‘I agree. The finale?’

‘Yes. Clapping and cheering and, these days, with such a large foreign element in the audience, you tend to hear whistles and squeals of a very un-French nature. And that theatre is the largest in Paris. There must have been close on two thousand people creating a din. Now, if his companion for the evening had been there during the murder she would have been an accomplice or – if a witness – would have been, I presume, made off with – eliminated? – by the guilty party. In some other place, at some other time, as there were no signs of further violence in the box, I understand. I would fear for the young lady’s safety, wouldn’t you?’

‘Accomplice? Witness? Not necessarily,’ said Joe. ‘She might have been the killer. What would you say?’

‘A woman?’ The doctor was taken aback. ‘Physically it’s certainly possible, I suppose . . . if she approached him from behind as I’ve demonstrated. You’d need a considerable rush of energy – determination, hatred . . .’ His voice tailed off doubtfully.

‘You don’t like the theory?’

Moulin smiled. ‘No more than I observe you do, Commander! We both know this is not a woman’s method.’

‘True. In my experience, when women plan a murder – and from whatever rank of society they come – they choose more subtle methods. Poison and the like. Anything from rat poison to laudanum. When the killing is done on the spot and the result of an overriding urge, or a desperate attempt at self-protection, they use the nearest weapon to hand – usually a domestic tool which, depending on their circumstances, may be a frying pan . . . a silver sconce . . .’

‘Contents of a theatre box not much use, I’d have thought. Could you throttle someone with all that gold braid?’

‘I wouldn’t want to try it. No. Someone chose to take this dagger into the box and use it. And leave it behind for all to see. This particular dagger. It’s distinctive. Meaningful. Personal, I’d say. The victim had fought in Afghanstan, his fellow soldier tells me. There’s a possibility that it may be from his own collection. Carried there by the victim himself and turned against him in an unpremeditated attack?’ Joe sighed. ‘Much work to be done yet, I’m afraid.’

Rising from his chair, Joe was struck by a sudden thought. He walked over to the corpse and lowered his head to sniff the improbably dark hair. He looked up and said: ‘Pomade?’

Moulin joined him and repeated the process. ‘Certainly,’ he agreed. He sniffed again. ‘Unpleasant. Not French. Much too heavy. I’d say something like Bay Rum, wouldn’t you? And it’s sticky.’ He took off a glove and tested a strand of hair between thumb and forefinger.

Joe did the same. He peered at the crown of the man’s head. ‘Well-barbered hair though a little long for most tastes, I’d have thought. Plentiful and would give a very good grip to anyone choosing to sink his fingers into it. As you demonstrated. Left parting and – look – it’s disordered on top. Could have happened involuntarily at any moment after the death of course, during the manhandling of the body by the authorities. But if your theory’s right, doctor, the killer must have had a disgustingly sticky left hand – and not sticky with blood. It’s not much but . . .’