“Get down!” Costa screamed again, waving his hands, waving the small black revolver through the falling cloud of white flakes. “Now! The bastard’s got a gun.”

He heard the first shot drown out the end of his warning. Something small and deadly sang its way through the air. Sparks flew off the column close to the astonished faces of the two cops under the portico. Falcone’s arm went out and pushed Peroni down to the stone pavement.

Costa was focused on the man on the steps now. The figure was directly by the fountain, dressed in black from head to foot and wearing one of those idiotic tie-down hats with earpieces that made you look like Mickey Mouse caught in a storm. He was standing in a professional firing position, the Weaver stance, right hand on the trigger, left supporting the gun, feet apart, comfortable as hell in the sort of pose Costa sometimes saw at target-shooting events. The small pistol was aimed, very deliberately, in their direction. A tiny flame lit up the barrel as Costa watched and a muffled crack rolled their way.

Costa scanned the piazza, doing his best to check there was no one else in the vicinity, then unleashed two shots towards the figure in the snow. A stream of tiny fires lit up furiously in response, sending more sparks up from old stone that was, at least, some kind of protection for them. For those who were smart enough to take it.

Mauro Sandri was still standing. Maybe it was panic. Maybe it was just second nature. The photographer was flapping around like a wild man, one hand still on his stupid camera, loosing off shots of anything, the Pantheon, the night, the three cops trying to squirm their way out of the firestorm coming at them from the square.

Then he turned, and Costa knew precisely what would happen next. Mauro spun round on his little heels, camera in hand, the motor drive of the Nikon clicking away like a clockwork robot, turned and faced the black figure still upright on the steps.

“Mauro,” Costa said quietly, knowing there was no point.

He was a stride away from the little photographer when the bullets hit. Two. Costa heard the reports as they left the barrel of the gun. He heard them hit the diminutive black figure on the steps, tear through the fabric of his winter jacket, bite like deadly insects deep into Sandri’s body.

The little photographer flew into the air like a man receiving an electric shock, then fell in a disfigured heap onto the ground.

“See to him!” Costa yelled at Peroni and Falcone as they scrambled to their feet in the deep, consuming snow. “This son of a bitch is mine.”

Knowing the idea was pointless, that no one could shoot that well, not even the dark-hearted bastard still standing by the frozen dolphins and fauns, Costa fired all the same, then began to sprint, began to hit his speed, and thought: At least I can run. Can you?

The figure was folding on himself, turning, like a crow shrinking into a crouch before it half jumped, half fell off a fence. There was fear there as he fled down the steps on the far side of the dolphins and fauns. Costa knew it and the knowledge made him run harder, heedless of the slippery, centuries-old paving stones beneath his feet.

He loosed off another shot. The man was fleeing into a corner of the square, trying to find sanctuary in the dark, tangled labyrinth of narrow streets and alleys that lay beyond, in every direction.

And, just as Costa was beginning to digest this thought, the weather joined in. A sudden vicious squall careered straight out of the north, a thick wad of snow lurking deep in its gut. The cruel, cutting ice stung and blinded. His feet gave way. The rugby player in him surfaced from the long-dead past, told him there was no option but to roll with the fall, to tumble into the soft, freezing blanket on the ground, because the alternative was to pitch gravity and momentum against the weakness of the human body and snap a tendon or a bone along the way.

It was dark and cold as he fell into the soft, fresh snow, striking the hard stone beneath with his shoulder. For a brief moment the world was a sea of whirling white and sharp, violent pain. Then he was still, feeling himself, checking nothing was broken.

When he got his equilibrium back and forced himself painfully to his feet, the figure in black had vanished. Dense clouds of white were falling with an unforgiving force again, burying the man’s footprints with every passing second, turning everything into a single, empty shade of nothingness. Costa strode to the corner of the street. The shooter could have gone one of two ways, west down the Via Giustiniani, towards the church of San Luigi dei Francesi, with its Caravaggios and, for Nic Costa, some bitter memories. Or north, into the warren beyond Piazza della Maddalena.

Costa stared at the ground. It looked like a fresh bedsheet, scarcely crumpled, full of secrets, all of them unreachable.

Reluctantly, knowing what he would find, he retraced his steps to the portico. A siren was sounding somewhere in the wintry night. Costa wondered how long it would take the ambulance to make its way through the treacherous streets. Then he saw Gianni Peroni hunched on the flat stone of the portico, hands over his eyes, next to Mauro Sandri’s inert form, and he knew it didn’t really matter.

He walked over, determined to handle this well.

“Hey,” Costa said, placing a hand on his partner’s shoulder, then crouching to peer into those strangely emotional squinty eyes, now liquid with cold and a bright inner fury. “We couldn’t have known, Gianni.”

“I will remember to point this out when I break it to his mamma, or his wife or boyfriend or whoever,” Peroni replied bitterly, trying to bite back his rage.

“He must have looked like one of us, I guess. It could have been you. Or me. Or anyone.”

“That’s a comforting thought,” Peroni mumbled.

Costa glanced at the dead photographer. Blood, black under the moonlight, was caking in Sandri’s open mouth. Two more patches, one on his upper chest, the second in the centre of his abdomen, gleamed on his jacket. Costa remembered that curious stance the gunman had held while firing at them. It contained some meaning. When they had started to swallow down the bitter bile of their shock, when the investigation proper began, this was a point to note, an item of interest to be pursued.

Peroni patted Sandri’s motionless arm. “I told him, Nic. I said, ”Mauro, you’re not going to die. I promise. You’re just going to lie there and wait for the medics to come. Then one day you go back to photographing mugs like me, and this time round you can take snaps of my pecker as much as you want. This time round“… Oh shit.”

“We’ll get the bastard, big man,” Costa said quietly. “Where’s Falcone?”

“Inside,” Peroni said with a slow, deliberate venom. “Maybe he’s enjoying the view.”

A blue flashing beacon began to paint the walls on the far side of the square. Then a second. The caterwauling of the sirens became so loud that lights came on in apartment windows in the neighbouring streets. Costa straightened up. There was no point in talking to Peroni when he was in this mood. He had to wait for the storm to pass.

Costa walked through the doors towards the stream of white that fell, still circling around itself, from the vast open eye of the oculus.

The caretaker was in his cabin by the entrance, florid face tucked into his chest, trying as hard as he could to stay out of everything. Leo Falcone stood by the inverted funnel, which kept growing as it was fed from the sky. Costa remembered studying the Pantheon at school in art class. Here, at the centre of the hall, lay the defining focal point of the ancient building, the axis around which everything was arranged in a precise show of ancient symmetry, both the great hemisphere and the monumental brick cylinder which tethered this imaginary cosmos to the ground.