My dog Nux spent most of the time hiding in the little room that I was turning into a masculine study. As soon as I could, I joined her. We were both there, doing nothing much, when Helena looked in, threw a nut at me, and said Petronius had just arrived. He had been invited with Maia, who was still being stand-offish, but had come with Ma and had stayed on. After he grabbed food and drink, Petro took me aside. He told me what he thought of my wine; it did not take long.

'It's leftover primitivum I cadged from Junia. And before you say it belongs to the cohort then, this will pay me back for the bribe I handed over to Rubella for help at the Quadrumatus house.' 'Oh we drank your cash up yesterday!' grinned Petro. 'That was for next year's party.' 'Nuts. As a bribe it didn't cover the aggravation that you've handed us at that villa.'

We settled in for a discussion. 'Look, Petro, it's all very well saying there's no crime. My view is that Mastarna let Scaeva die – genuine accident, maybe – but then Mastarna is unlikely to have decapitated the corpse. For one thing, if he did, he's just a hired man and the Quadrumati would have had no compunction in exposing him. No, they are trying to shield one of their own. I am sure the freedwoman, Phryne, was malevolent enough to grab a knife and do the deed – and then she carried the head to the pool.' I remembered now, how she had looked when I asked whether weapons or treasure were found in the atrium pool with the head: Should there have been? 'Even if that's all she did, somebody needs to tell Quadrumatus to stop looking away and deal with the woman. I thought I might write to Rutilius Gallicus and make him responsible for stiffening up his so-called friend.'

Petronius shrugged. 'Well you do that, and I'll get Rubella to ram home the message too.'

'I think there was more to it, Petro. I think that the poor flute boy saw what she did. The family covered it up but he was terrified of her. That's why he ran. When he was brought back to the villa, he may have become hysterical; Phryne killed the boy to keep him quiet.' Petronius looked troubled. 'It's not her.' 'Alibi?' 'Her mistress vouched for her… Surprised? I'm still baffled by this flute boy death, Marcus. Scythax is being a menace over it – he is sticking to his theory, that the boy was killed like the street vagrants. The freedwoman can't have been constantly out of the house at night, killing runaways. I've explained to Scythax that the boy was found dead by you, indoors, and it just doesn't fit. Scythax wants to do more work on the corpse, but the Quadrumati won't allow it -' 'I told you; they are covering. They don't want a scandal.' 'Well, Scythax is rambling. There can't possibly be a link between that villa's household and what's happening to runaway slaves on the streets of Rome. We're stuck, Marcus.'

I had reached the mellow stage by then. 1 told him we could think about the flautist tomorrow, when everything returned to normal. Most likely, since there was nowhere else to go with the case, we would have to forget about it.

The night went on. Pa and some of my sisters went home. Zosime returned to her temple. 'Will you continue your work with the homeless?' Helena asked her as we bade farewell. 'Oh yes. I've been doing it ever since 1 was first trained.' 'Well, good luck to you!' A few favoured people remained and we would probably stay up for hours yet; it was the soldiers' last night with us and they were melancholy to be losing domestic comforts. 1 sat fairly cheerfully among my family, waiting for the next angrily slammed door, the next whining child with a sore throat, the next tipsy woman to tread on the dog's tail…

1 thought 1 was cheerful, but melancholy thoughts came drifting through my brain. 1 found myself thinking about the runaway who had told me his life story on the Via Appia – the ex-architect with the long tale of woe. 1 had learned that man's whole history, yet never even knew his name. 1 would never see him again, never know his fate. He had been sickly and could by now have died of December cold. His run of bad luck could even have ended with a final gasp, strangled by the unknown killer who bent over sleepers in doorways and choked the life out of them. 1 wished 1 could have asked him if he had ever seen the killer at work.

Then, as the oil lamps flickered and wine wafted me halfway to oblivion, the truth hit me: Scythax was right. There was a link between the villa and the dead runaway slaves. The flute boy may have been killed at Phryne's instigation yet it was not one of the household who took his life, but somebody who came in from outside. One of the doctors employed by the Quadrumati had let a patient bleed to death by accident. That was nothing; another was far more menacing.

1 ordered Justinus to stop smooching Claudia and come with me after Petronius, who had left to go on duty at the patrol house. Once there, 1 asked Petro if his famous lists of undesirables included doctors. Since medicine is akin to magic, he had a list all right. He would not let me see it, but he found the address we needed and we set off to apprehend the man whom I was now convinced must be the killer.

'He hates all slaves. I've heard him disparage them – Hades, he even sneered at me when he supposed I was one – and people have been telling me about his attitude ever since I first met him. He follows the same broad Hippocratic doctrine as Zosime and the doctors at the Temple of Жsculapius. Zosime, or maybe it was someone else, told me a long time ago that he trained her. She calls the way they work, "softly, safely, sweetly" – but he has foully perverted that…' We were going to see Cleander. The streets were a nightmare, full of revellers who could not understand our need to pass through the crowds quickly. Petro had brought a few men, but most were too busy attending fires that night. The smell of smoke hung on the air, as thickly as the noise of merriment. We found the house. It seemed to be in darkness, but after muted knocking by a vigilis who pretended to be a patient, Cleander himself opened the door.

Petronius Longus led him back inside and began to interrogate him. In response, Cleander only glared haughtily. We were all beneath him. He treated the charge of murdering the runaways with chilly contempt. Soon he began refusing to answer any questions at all. Petronius eventually had him taken away to the patrol house.

'Seen it before, Marcus. He will never confess. I can put Sergius to work on him, but this man is so arrogant he will think it a challenge to withstand the pain.' 'Maybe his slaves – or his patients – will give up information.' 'I bet they'll protest his innocence just as much as he does.' 'All his patients thought he was wonderful.' 'And his household won't admit that they should have seen what he was doing.'

'Well, keep at it, lad. If you let it be known among the vagrants that he's in captivity, you may just find more witnesses. His activities were known among the runaways, but fear kept them silent. Even Zosime should help. He trained her, but I never had the impression she was particularly loyal to him. She hates what has been done to the runaways, for one thing. Shock her with the facts; she'll give evidence. '

Petronius was called away. He left a man to guard the house, ready for a full search of the property next day. Justinus and I cast a quick eye over various rooms, and were about to leave ourselves. Then the vigilis called to us; he had found a locked closet. We could not discover a key to it; Cleander must have taken it. For half a beat we nearly left it for the lads to search the following day. But in the end, Justinus put his shoulder to the door and forced it open.

The interior was in darkness. As we crashed in, a faint groan alerted us to human presence. We ran for lights. Then we saw that Cleander had left a patient, or a victim, strapped to a pallet. He was gagged, and blood trickled inexorably from his arm into a by now very full bowl.