She threw bag after bag of ballast over the side and watched them thunk onto the cobbles far below. Not far enough below for her liking. Streets and squares and parks unfurled beneath them, sprawling out from the prison like a spider’s web. A muddy swirl of slate roofs and green treetops washed up against the brown ribbon of the Seine as it flowed through the centre. The river split around the Île de la Cité, where the Notre Dame cathedral dominated at one end. At the other lay the prison, among the complex of law courts and Revolutionary headquarters. Above them, a broad sky yawned the cool eggshell blue of a half-hearted summer.

The tear in the balloon flapped jauntily in the wind, huffing out precious hot air. Ada had thought they’d been too high up for the prison guards’ musket fire to reach them.

She’d been wrong.

In the distance, she could just about see the Place de la Révolution, where the guillotine lay waiting. When the Revolution had started five years ago, it had been called Place Louis XV, named for the previous king. There had been hope of building a new France, of finding a better, fairer way to rule. But the new government had floundered, and King Louis XVI had been executed in the same square. It changed everything. It was as though France was a frustrated child, finally getting what it wanted, but finding the prize sour and disappointing. Without the king, people still starved, inequality continued. The country splintered and the different factions spat at each other like a serpent with many heads. In the middle of all of it, Ada, Al and the rest of the Battalion of the Dead were the last port of call for anyone with a loved one in trouble – whatever side they were on – with prison breaks their speciality.

Ada loved the thrill of the chase, the flare of pride when a plan came off. But sometimes she knew they pushed their luck too far. The hiss of escaping air gave her the creeping fear that this was going to be the last mistake she ever made.

‘What do we do?’ shrieked Al, peering over the edge at the crenellations of the prison that were rapidly rising to meet them.

Ada rammed a hairpin back in place to secure the tight, black curls that framed her brown face. She threw another couple of ballast bags overboard, giving a calculating glance at the ground to try to judge their rate of descent. Dread made her chest tight, but she was damned if she’d let Al see it.

‘I’m making us lighter,’ she explained. ‘It’s a basic scientific principle. Hot air rises – we have lots of hot air above us – but only if we’re lighter than the volume of air.’