There. Two windows changed shape; two patches of darkness grew and gathered form. Two figures leaped from the first floor windows directly overhead, hurtling down to land upon the platform. Twin thuds of booted feet.

Only Lockwood and I saw them; everyone else was fixated on the ghosts. For a split second I had a clear view of the man nearest to me. He wore black sneakers, faded jeans, a black zip-up top. His face was hidden behind a black ski mask, but through the mouth-hole poked an overhang of bright white teeth. In one hand he had a rapier; in the other a snub-nosed gun. Strapped across his spindly chest—his top was half unzipped—a leather belt held odd devices. They looked like short batons, the kind relay runners use, with clear glass bulbs at one end. Pale light swirled in them. I knew what they contained.

An instant’s glimpse; then he was away, he and the other man, flitting across the platform. They went toward the front of the float where Penelope Fittes stood in her bright white coat, a crescent dagger in her hand.

Lockwood and I ran, too; but we were too far away to intercept them.

As they drew close, the nearest raised his gun.

Lockwood threw his rapier, hard and horizontal, like a javelin; it nicked the assailant’s arm, knocking the gun away.

Then I was on him, striking left and right. He parried my blows with quick defensive moves. It told me he was agent-trained.

The other man ignored us. He walked quickly toward Penelope Fittes, reaching in the pocket of his jacket. Now his hand too held something small, snub-nosed, and black.

Ms. Fittes saw it. Her eyes widened. She fell back against the rails.

The edges of the float were decorated with plastic lions and unicorns. Lockwood grasped a unicorn by its horn and snapped it off its pole.

The assailant aimed his gun—

Lockwood dived forward, swinging the unicorn in front of him.

Two blasts; two thuds, so close in time they became one thing, a start-stop noise. The unicorn spun out of Lockwood’s grasp, a pair of neat round holes halfway up its neck.

Now the man I was fighting brought his greater strength to bear; his swordplay became faster. His blows jarred my rapier in my hand.

All at once he stopped, looked down in some surprise. I was surprised too. He had a sword point poking through his chest.

The man swayed, then toppled sideways. Behind him Mr. Rotwell pulled his blade clear.

The remaining assailant had turned toward Lockwood. But now from the other side Sir Rupert Gale strode forward, rapier raised and moving fast. The man paused, fired at Sir Rupert, and missed. With a spring, he bounded away along the platform.

Lockwood was scooping up his rapier. “We can catch him yet, Luce,” he cried. “Come on!”

We ran along the platform, almost empty now. Past George, busily subduing the Visitor with salt and iron; past Holly, tending to the fallen. The second ghost had vanished, destroyed by Rotwell’s agents. Mr. Rotwell and Ms. Fittes themselves were left behind.

The man in black reached the end of the truck, gave a mighty spring, and landed on the cab of the following vehicle. Lockwood leaped after him, coattails flapping; a moment later I did the same.

Over the cab roof, boots clattering. Onto the show float, racing under gothic arches, cutting through a confusion of screaming performers. The assailant swung his sword, fired his gun in the air. Men in ghost-sheets and women in long bloody dresses took running jumps over the edge in clouds of talc, landing in the crowds like phantoms from on high. Tidal waves of terrified yells rolled out around us. The man in black turned, aimed the gun at us; it didn’t fire. He tossed it away, kicked at a foam arch, sent it tumbling down. Lockwood dived one way, I the other. It crashed between us, squashing a small actor.

Running jumps to the next float, decorated with the mustard hues of Dullop and Tweed. Above us loomed their papier-mÂché symbol, a giant all-seeing owl. The assailant threw a flare; it burst against the owl, tearing a hole and sending a rain of burning matter onto our heads.

Lockwood and I didn’t break our stride. We ducked, brushed hot embers from our hair, ran on.

It was the official Rotwell float, the next one, and again it was close enough for the fugitive to leap to. Here were scattered piles of plush lions, Rotwell soft drinks, and other gifts to be given to the crowds. The agents who’d been in charge of them were gone. The man in black slipped and skidded on the toys and bottles; with a curse he turned, hurled down a ghost-bomb. A willowy figure rose up—and was instantly sliced to ribbons by simultaneous slashes of our swords.

We were almost upon him, so close I could hear his ragged breath. He reached the back of the float. Beyond was a gap, impossible to jump: the next float was many yards away.

“Got him,” Lockwood said.

But there, at the end of the truck: the Rotwell lion, a giant helium balloon straining at the end of a tethering cable. The man in black slashed the rope free, caught it as it whipped away. He was carried up and out over the Strand. He tossed his sword clear; now he dangled from two hands.

Lockwood and I slammed into the side of the truck. Lockwood exhaled. “Drat. Not sure I can quite match that.”

“It’s blowing him toward the river.”

“You’re right, it is. Come on.”

Down onto the street, amid the deserted stalls and sideshows. A moment before, a mass of people had stood here. Now it was a field of hats and lavender, of scattered charms and abandoned shoes. The Poltergeist rides had stopped mid-session; trapped customers called to us from atop the extended arms. Down the road, off the Strand, and up the gentle rise to Waterloo Bridge, Lockwood and I kept running, side by side.

I glanced across at him—his eyes were bright, his face set, his long legs swinging beside mine. We were in step together, perfectly in sync. And in that moment the world around us dimmed and blurred. Tensions and disagreements fell away. Everything was simple. It was just us, together, chasing a giant helium lion down a central London street. Everything was as it should be—back in its proper place.

Perhaps Lockwood had had similar thoughts. He grinned at me; I grinned at him. A swell of joy rose in me, displacing the ache of my muscles, my burning lungs. It was like the last few weeks just hadn’t happened. I wanted it to last and last—

“Hope I’m not disturbing anything.”

Drawing abreast of us, sword-stick moving easily: Sir Rupert Gale, punctiliously polite as ever. If he’d had a hat, I bet he would have raised it as he ran.

“Hello.” I didn’t strictly want to answer, but his courtesy was catching. “This fellow’s a trier, isn’t he?” Sir Rupert nodded at the precariously dangling form ahead of us; the river’s breeze had caught the lion, which was now being buffeted dangerously. The man was being dragged, bumping against a wall. “I swear I almost want him to get away.”

“Getting away from you is a tough art,” Lockwood said. “I bet only the very best can do it.”

“Ha, ha! Yes!” Sir Rupert Gale smiled as he ran. “He’s going to go out above the river. If I had my Purdey twelve-gauge shotgun with me, I’d take a potshot now and chance it. He’s not so high the fall would kill him.”

He had no gun, and we could not run fast enough. Even if we had, the balloon was still too high to catch. It floated out above the bridge. For a moment the Rotwell lion was beautifully illuminated by the lanterns on the parapet, sparkling like a Christmas bauble on a child’s tree. We saw the man clinging desperately below it, still masked, his jacket and shirt pulled up so that his pale back and stomach were exposed. Strong winds took him; the lion was whirled around. I thought it might be looping back toward us. Then it was pulled out to the center of the river, and that was when the figure lost his grip and fell, thirty, forty feet, into the black Thames. He hit it hard. The waters closed over him. We ran to the balustrade, the three of us, and craned our necks, but saw nothing.

Minutes passed. The lion balloon was already almost lost to sight, a glittering point of red carried by river winds east toward Blackfriars Bridge, the Tower, and, ultimately, the sea.

“Dead and drowned, I suppose,” Lockwood said.

Sir Rupert nodded. “You’d think. Then again, we all know better than that.” He tapped gloved fingers on the balustrade.

I stepped away. “Who were they?” I said.

“Enemies of Fittes and Rotwell, presumably,” Lockwood said. “Anyway, he’s gone.”

“Yes.” Once again Sir Rupert Gale tapped his fingers on the stone. He turned away from the edge and, in the same deftly casual movement, his rapier flicked up and jabbed straight out at Lockwood’s side. The action was so quick I didn’t fully comprehend it; nor the way Lockwood’s arm shot down to block the sword tip with his rapier guard. For a second, the blade was caught in it, trapped in the twisted fronds of metal; I could sense Sir Rupert’s exertion, Lockwood’s, too. It gave me a chance to see how close the sword had been to slicing cleanly beneath the ribs. It would have traveled into his lungs and pierced his heart. Then the young man sprang back, wresting the tip free. His eyes were bright, he balanced lightly on the tips of his toes.

“Fast,” he said. “Well done.”

“You, too.” Lockwood turned to face him, bending his wrist as if it pained him. “Of course, I never attack from behind.”

“Oh, hardly behind, Mr. Lockwood. You had a fair chance, as you’ve just proved most admirably.” Sir Rupert ran a hand through his hair. “Well, our mutual enemies have gone, as you say, but here we are—you and I alone together. Isn’t this a wonderful opportunity to settle our dispute?”

“Hey,” I said. “In just what sense is he alone? I’m here too.”

“Don’t worry, Luce,” Lockwood said. He flicked back the edge of his coat and lifted his rapier. “Well then, Sir Rupert? Come on.”

“You can’t do this!” I cried. “There’ll be witnesses! The others will be here in five minutes—”

“Miss Carlyle,” Sir Rupert Gale said, “a few seconds is all I need.”

Lockwood’s grin was flinty. “That was what I was going to say.”

Shouts and beams of swirling flashlight. Up along the crest of the bridge came George, followed by a host of Fittes and Rotwell agents. Lockwood and Sir Rupert Gale stood looking at them. Then Sir Rupert laughed and returned his rapier cleanly to his belt.

“And now we’re heroes together,” he said. “What an experience. What an excellent evening.”

He smiled at us; we smiled at him. Three crocodiles on a muddy shore could not have smiled at each other more eloquently or with such gleaming teeth. We stood waiting, the three of us, and a moment later were engulfed by shrill inquiries and breathless congratulations.

In the aftermath of the carnival attack, certain things swiftly became clear. Other things did not.

Remarkably, only one person had incontrovertibly lost his life—the assailant killed at Mr. Rotwell’s hand. The body of the other, despite police (and relic-men) combing the Thames shoreline the next day, was never found. Unlikely as it seemed, it was possible he had escaped.

Within minutes of the attack, the Strand and surrounding streets were sealed off, and the grand parade abandoned. Twelve people, eight from the crowd and four from the Fittes and Rotwell float, had suffered ghost-touch. All were treated on site by medics traveling with the parade. Speed of response ensured that all of them pulled through—even the tweedy lady first enveloped by the Visitor. She had been kept alive by an adrenaline injection administered on the spot by Holly Munro.

George had single-handedly subdued the original ghost. After surrounding it with iron, he had hunted across the platform till he found the splinters of broken glass that marked where the missile had struck. There too he found a piece of jawbone, complete with two brown teeth. When this was wrapped in silver, the Visitor had vanished. Further exploration by other agents located five other Sources scattered among the debris of the floats and street.

Penelope Fittes was uninjured. Steve Rotwell had sprained a wrist while helping his operatives subdue the second Visitor. Both leaders appeared in a photograph on the front cover of the Times the following day, Rotwell’s arm displayed prominently in a monogrammed sling.

Curiously enough, despite ending in complete disaster, the carnival—from the point of view of the authorities, at any rate—was a notable success. The shock of the attack seemed to bring the people of London to their senses. Perhaps it was the very human nature of the assassination attempt. Perhaps it was outrage at the real physical danger Ms. Fittes and Mr. Rotwell had been in. Present difficulties notwithstanding, they were icons, representatives of the noble firms that had done so much to keep the population safe for over fifty years. Whatever the answer, after that night the Chelsea protests more or less evaporated. DEPRAC and the agencies were left to go about their business undisturbed.

One other immediate result of the events was a new celebrity status for Lockwood & Co. A photograph of Lockwood during the chase appeared on page three of the Times, and in several other papers. He was caught mid-jump between two floats, his coat flying out behind him, his hair blowing back, his sword held so loosely in his hand it seemed he scarcely touched it. He was a thing of light and shadow, fragile and dynamic like an airborne bird.

“That’s one I’m definitely putting in the album,” George said.

We sat in our living room, bottles of lemonade on the table, glasses in our hands. The fire was on; we had the curtains shut against the dying day. Piles of crumpled newspapers lay between us, scrutinized and cast aside; it almost seemed like our old habits of mess and squalor were back again. Holly Munro had been too busy to worry about it. She’d been fielding calls all day. She was with us now, our casebook open on her knee. Up on the cabinet, the skull in the ghost-jar, quiet and unnoticed, overlooked the happy scene.

“Oh, I shouldn’t bother really, George,” Lockwood said. He took a sip from his glass. “Though if you do, the one in the Guardian’s got the nicest resolution. They don’t crop the coat like the Times does, either. Plus, you get a bit of Lucy’s knee as well.”

I snorted good-naturedly. My knee aside, I wasn’t in any of the published photos, but for once the papers had mentioned me by name. In fact, all of us got in. My action against the assailants; George’s struggles with the ghost; Holly’s life-saving efforts with the syringe: all this had been noted and praised. But Lockwood, who had protected Ms. Penelope Fittes at the crucial moment, was the one singled out for the highest commendation. Certain rich industrialists who had been on the beleaguered float were quoted as mentioning awards.

“We’ve had so much interest since last night,” Holly Munro said. “Requests for interviews, and many possible cases. All of them thanks to you.”