Irene and the Persian had doffed their Khasi and Princess disguises. Now, they wore close-fitting black bodystockings with tight hoods like those popularised by the English soldiers at Balaclava. The lower parts of their faces were covered with black silk scarves; only their eyes showed.

They crept along the deck of the barge, conscious of the music and chatter below. The clowns were performing some interminable rhapsody from Bohemia, which made Irene vow to avoid that region in the future. The full moon and the lights of the city were not their friend, but they knew how to slip from shadow to shadow.

On the Pont du Carrousel, a solitary man stood, looking down at the dark waters and the barge. Irene saw the shape and laid a hand on the Persian's arm to stop him stepping into moonlight. They pressed against the side of a lifeboat, still in the shadow. Irene first assumed the man on the bridge was a stroller who had paused to have a cigar, though no red glow-worm showed. She hoped it was not some inconvenient fool intent on suicide – they did not want attention drawn to their night-work, with lanterns played across the water's surface or the decks where they were hiding. The figure did not move, was not apparently looking at the barge, and might as easily have been a scarecrow.

Irene slipped away from the lifeboat, did a gymnast's roll, and found herself next to the housing of some sort of marine winch. Heart beating fast, she looked up at the bridge. The possible spy was gone. There had been something familiar about him.

The Persian joined her.

The Countess Cagliostro's barge was armoured like a dreadnought. That was why it sat so low in the water. Aft of the ballroom were powerful engines, worked by humming dynamos. The barge was fully illuminated by electrical Edison lamps, and mysterious galvanic energies coursed through rubber-clad veins, nurturing vast sleeping mechanical beasts whose purposes neither of Erik's operatives could guess.

'She could invade a country with this thing,' said Irene.

'Several,' commented the Persian.

'Do you think it's a submersible?'

The Persian shrugged. 'I should not be surprised if it inflates balloons from those fittings, and lifts into the skies.'

'You've an inventive turn of mind, pardner.'

'That is true. It is part of the tale of how Erik and I became associates, back in my own country… but this is not the time for that history.'

'Too true. Let's try and find the lady's lair.'

Beyond the engines, the deck was a featureless plate but for several inset panes of thick black glass. Irene reckoned this was Erik's trick again – transparent for the sitting spider, opaque for the unwary fly.

From the pouch slung on her hip, she drew a cracksman's tool: a suction cup with an arm, attached to a brutal chunk of diamond. The tool was worth more than most of the swag Irene had used it to lift – the cutting gem had been prised from a tiara and shaped to order by a jeweller who nearly baulked at the sacrilege of turning beauty into deadly practicality.

Irene cut a circle out of the glass, and placed it quietly on the deck.

The space below was dark, a pool of inky nothing. Working silently, the Persian unwound a coil of rope from his torso and made a harness for Irene. After a tug to test the line, Irene stepped into the hole and let herself fall. The Persian, anchored strongly, doled out measured lengths of rope, lowering her by increments.

Once inside, the hole above was bright as the moon, and all around was cavernous dark. Irene blinked, hoping her eyes would adjust – but the gloom was unbroken, the dark undifferentiated.

Then there was a musical roaring, as if a steam calliope were stirring, and a thousand coloured jewels lit up, dazzling her. Incandescent lamps fired and Irene found herself dangling inside what might have been the workings of a giant clock. Gears and wheels, balances and accumulators were all around, in dangerous motion, scything through the air. She had to twist on her rope to avoid being bashed by a counterweight.

Music played – mechanical, but cacophonous, assaulting her ears.

The Persian began to haul her upwards hastily, out of the potential meat-grinder, and she climbed, loops of rope dangling below her. A razor-edged wheel whirred, slicing through loose cord.

Irene was pulled up on deck. By more than two hands.

Light streamed upwards from the hole.

Men in striped jerseys caught her. Their faces were covered by metal half-masks. The Persian, scarf torn away and hood wrenched off, was held by a stranger character, one of the ten-foot toy soldiers from the ballroom, miraculously endowed with life. Its tin moustache bristled fiercely and its big wooden hands gripped like implements of torture. Slung on its back was an oversized musket with a yard-long bayonet. Stuck out of its side was a giant key. The Persian was lifted completely off his feet, crushed against the soldier's shiny blue tunic.

'Messieurs,' said Irene, 'you're taking liberties. Get your paws off the goods if you don't intend to buy.'

The half-masked sailors were briefly confused, and relaxed their ungallant grip on her person. Irene darted and her slick leotard slipped through the hands of her would-be captors. Like an eel, she was out of their grasp, heading towards the side of the barge. If she got over, she would have a chance. The Persian could be rescued later, if that were possible.

Something rose from the shadows and took a much faster hold.

Three swift blows to the stomach knocked the wind out of her. She doubled up in pain, and was recaptured. The sailors were less considerate about keeping hold of her now.

The thing that had struck her emerged into the light.

It was a woman – of course – wearing a costume modelled on Elizabeth of England, with a red lacquered moon-face mask and towering headdress. Dozens of pearls studded bodice and face, exciting Irene's larcenous instincts. Getting her breath back, she sighed at such extravagance.

'The Countess Cagliostro, I presume.'

'Your hostess,' said the woman. 'Though I don't remember putting your names on the guest list. What were they again?'

'I'm Sparkle and he's Slink,' said Irene. 'We're desperate apache thieves. You've bushwhacked us properly, so do us the courtesy of summoning gendarmes and handing us over to French justice so we can start plotting our escape from Île du Diable. We accept this as an inevitable reverse of our chosen profession, sheer crookery. And there's no need to be unpleasant about it.'

The Countess's mask seemed to smile, its eye-slits narrowing.

She glided, on invisible feet, to the side of her toy soldier, and twisted the key as if winding a clock. Then she stood back, and the key turned as – with big, jerky motions – the soldier raised the struggling, bleeding Persian above its head, then dropped him over the side of the barge. After a long scream, there was a splash.

Irene's heart leaped. This was not what had been planned.

The soldier stumped away from the edge of the barge, and the Countess paid attention to Irene.

'Now that's taken care of, let's talk about you.'

Irene deemed it politic to swoon.