‘Or how to complain,’ said Michael drily.

‘Thank you, medicus.’

V to IV, and here was another deck Stef was familiar with, the ‘barracks’, the level where she had first boarded the ship. There were orderly rows of huts here, accommodation for the century of legionaries and the various auxiliary units that made up the ship’s military force. Titus pointed out a group of huts, almost an afterthought in the layout below, where the remiges were quartered when off duty, the ship’s crew, all of them Brikanti. Away from the obviously military facilities were blocks of housing, clustered around squares and courtyards. Here Stef could see women working and walking, a huddle of children engaged in what looked like some open-air lesson. She was reminded that these soldiers had brought their families with them on this interstellar march, their wives and lovers, and children born in and out of wedlock.

There were legionaries stationed at the hole in the floor through which they would descend further. And this time the breach was actually blocked by a covering of wood and glass.

Michael dug into his satchel and handed Titus and Stef masks of linen soaked in some kind of alcohol. ‘You may prefer to wear this when we descend.’

Stef apprehensively donned the mask.

The platform slowed as it approached the level of the deck. Titus spoke softly to the guards stationed there, and they laughed at a joke Stef did not hear. Then the guards hauled back the big hatches that covered the portal in the ground, and the platform descended.

IV to III. The slave pen.

It was the stench that hit Stef first, a stench of shit and piss and vomit, of blood and of rotting flesh – a stench of an intensity she hadn’t known since her first experience of zero-gravity emergency drills, in her early days as a raw ISF recruit.

Then she made out the detail of the deck, sixty metres below. Illuminated by bright white light, the entire floor was covered by an array of cubicles, neat rectangular cells, block after block of them lapping to the hull on either side. Above the floor, supported by angular gantry towers and fixed to the hull, was a spider web of walkways and rails, a superstructure of steel. Soldiers patrolled the walkways, or were stationed on towers mounted with heavy lights and weapons. All the troops wore masks. The troops carried none of the gunpowder handguns they called ballistae, she saw; instead they were armed with swords, knives, lightweight crossbows. Even the big weapons mounted on the towers were some kind of crossbow. No high energy weapons in a pressure hull; it was a good discipline that the ISF had always tried to follow.

It almost looked neat, industrial, a cross between some vast dormitory and a beehive, she thought. Until she looked more closely at the contents of the cells.

What had looked like worms, or maggots perhaps, were people, all dressed in plain greyish tunics of some kind, crammed in many to a cell. She thought she saw bunks – or maybe shelves would be a better word. People were stacked, like produce in a store. A party was working its way along a corridor that snaked between the cells, hauling at a kind of cart – a cart laden with bodies, she saw, peering down, bodies loosely covered by a tarpaulin, with skinny limbs dangling from the edges.

Titus seemed moved to explain. ‘Obviously none of the slaves is allowed above this level because of the ongoing plague. So the security issues are more troublesome than usual.’

‘ “Troublesome”?’

‘We’ll find your slave boy. There’ll be a record of his cell.’ The platform was slowing, and Titus pointed. ‘You can see this shaft goes on down to the lower decks, but we’ll stop at the walkways and move out laterally from that point.’

For one second Stef bit her tongue. This isn’t your world, Stef. Keep out of trouble  … The hell with it. She turned on Michael, her self-restraint dissolving. ‘You’re supposed to be a doctor. Do you have the Hippocratic oath in your world? How can you condone this, how can you cooperate?’

Michael looked at her strangely. ‘You ask me? We Greeks think the Romans are soft on their slaves.’

‘Soft?’

‘There are ways for slaves to win their freedom, in much of the Empire. But to us the slaves are barbarians, irredeemable. Once a slave, always a slave.’

‘But you’re a doctor … Never mind. I guess my own people don’t have an unblemished record. You say there’s a plague down here?’

‘Yes. It is …’ The words Michael used were not translated by the ColU’s earpiece.

She dug her slate out of her tunic pocket. ‘ColU, are you there?’

‘Always, Stef.’

Of course he was listening in; she wouldn’t have been translated otherwise. ‘You have chemical sensors in this thing? Can you tell what the plague is from up here?’

Michael and Titus both stared as she held the slate high in the air, pointing the screen down into the honeycomb of a deck.

After a pause, the ColU said, ‘A kind of cholera, I think. Clearly endemic on the ship. I imagine that the appropriate vaccines are unknown to this culture. The disease must flare up when water filtering systems fail – it is possible the Romans don’t even understand the mechanism, why filtering is effective – and the death rate in the conditions you show me below—’

‘Am I in danger?’

‘No, Colonel Kalinski. The immunisation programmes the ISF gave you over the years leave you fully protected.’

‘And Yuri was surely treated too.’

‘By the ISF medics before he was left on Per Ardua, yes.’

She thought quickly. ‘Could you manufacture a vaccine? You could start from samples of our blood …’

The ColU hesitated. ‘It is not impossible. With the help of the medicus, perhaps, the assembly of a cultivation lab from local equipment … it might take time, but it could be done.’

‘In time to save a lot of lives?’

‘Yes, Colonel Kalinski.’

Titus put his big hand over the slate, gently compelling her to lower it. He said tensely, ‘You speak to your oracle through your talking glass. It perturbs me that my commanders seem willing to accept you and your miracles without explanation. I would not permit it, were I the centurion—’

‘But you are not, Titus Valerius,’ Michael said.

‘No. I am not. But I believe I understood what you have plotted with the oracle.’

‘ “Plotted” doesn’t seem the right word—’

‘You intend to damp down the plague, to preserve the lives of slaves who would otherwise die.’

‘That’s the idea. What’s wrong with that?’

Titus fumed. ‘It will break the ship’s budget, and bring us all to starvation long before we cross the orbits of Constantius, Vespasian and Augustus, that’s what!’

Stef frowned. ‘I don’t understand.’

Michael said calmly, ‘I fear you do not, Stef. You are not used to thinking like a slave-owner. I have mixed with the Brikanti, for example, who use slaves much less sparingly – indeed, mostly for trade with the Empire. But you are a star traveller. You must know that a ship like this has a fixed budget, of consumables, water and food and air.’

‘Of course.’

‘Then you must see that to the centurion – or specifically the optio who manages such things – the slave labour aboard is just another asset, to be used according to a plan. In the first year we have so many slaves, who will eat this much food, who will get this amount of work done – of whom this number will die of various causes, and in the second year we will have a diminished number of slaves, reduced by the deaths, augmented by births, of course, but most of those will be exposed. And that diminished number is in the plan, as is the food they eat, the work they will do, the further deaths during the year—’

‘And so it goes on,’ said Stef.

‘So it goes on,’ Titus said grimly. ‘And as long as there’s one slave left at the end of the journey to wipe the centurion’s arse, the job will be done.’