The warship swung towards the yacht and the two vessels closed quickly. Less than an hour later, a young Roman officer, who Cato recognised as one of Petronius’s tribunes, climbed on to the deck of the Sobek and strode towards Cato.

‘Urgent despatch from the governor, sir.’ The tribune held out a leather tube with the governor’s seal securing the cap. Cato took the tube, broke the seal and took out a small scroll of papyrus. Moving to the side of the ship, he unrolled it and read through it quickly, then again to be quite sure about the contents of the message. He rolled it up and gestured to his friend. ‘Macro, on me, please.’

‘What is it, sir?’ Macro asked in an undertone once he had joined Cato.

‘The Nubians have crossed the frontier. They’ve invaded Egypt. Petronius has ordered the Twenty-Second to advance up the Nile to Diospolis Magna. He aims to mass all his available forces there before moving against the Nubians.’

‘Which means he wants us to return to Alexandria at once, I take it.’

‘Yes.’ Cato clenched his fist round the scroll, crushing it. ‘It seems that we are obliged to abandon the hunt for Ajax.’

Macro’s heart felt leaden with disappointment and looking at his superior he could see that Cato shared his bitterness. Macro cleared his throat. ‘It’s just for now, sir. We’ll continue the job once the Nubians are dealt with. We’ll find the bastard, don’t you worry. He’ll pay for what he did to me, and to Julia. I swear it, on my life.’

Cato stared at him and nodded. ‘As do I.’

Then he took a deep breath and crossed the deck towards the tribune. ‘Tell the governor we are making full sail and will return to Alexandria without delay.’

‘Yes, sir.’ The tribune saluted, and then hesitated. ‘Is there anything I should report to him, sir? Any progress you have made in locating the renegade?’

‘No. There’s nothing,’ Cato admitted. ‘Now be off.’

The tribune went to the side and climbed down the rope ladder on to the deck of the sleek yacht. At once the crew fended it away from the side of the warship and raised the triangular sail. The wind filled the sail with a dull crack and the yacht heeled as it picked up speed and pulled away from the Sobek, heading west.

Cato turned to the trierarch. ‘Set course for Alexandria. Signal the other ships to follow us.’

‘Aye, sir.’

As the warship got back under way, Cato stood at the side rail, staring towards the coastline. Ajax was out there, somewhere, and free to continue wreaking damage along the delta, unavenged. It was a bitter thing to swallow, but there was nothing that could be done about it.