The ride back to Forty-Second Street had never felt so long. The subway seemed to be inching, crawling, oozing through the darkened tunnels, as if its wheels were coated with molasses. Thomas knew that on average, subway trains took three minutes to move from one station to the next, but it felt to him like three hours. Anxiety was crawling through his whole body, as if a thousand ants were marching under his skin. Every time the train stopped at a station and the doors slid open to admit a shuffling mass of passengers, Thomas had the urge to scream. Max bit her nails to shreds and Sam gripped one of the handrails so hard, he left an enormous dent in the metal.

Had they done the right thing by running away from Rattigan? He didn’t know. He couldn’t think clearly. For once in his life, his brain could produce not a single useful calculation or statistic. He couldn’t imagine what Rattigan wanted with them in the first place.

All he knew was that he would never forgive himself if anything happened to Mr. Dumfrey.

Finally, they were only one stop away. But halfway to Forty-Second Street, the train gave a jerk and a groan and shuddered to a stop. The lights flickered and then went off; the car was plunged into darkness. The passengers in the car began to mutter.

“Last week I got stuck right here for forty-five minutes,” someone said with a sigh. “Engine problems.”

“You gotta be kidding me,” Max said, but her voice was high-pitched, nearly hysterical.

“We’re running out of time,” Pippa whispered. “If Rattigan got to Dumfrey . . .” She didn’t finish her thought. She didn’t have to.

“Follow me,” Sam said. He pushed his way over to the side of the car, wedged his fingers into the little crack between the doors, and, with a grunt, pried them apart, grateful for the darkness, which meant that nobody could gape. “Come on!” He held the doors open as Thomas, Pippa, and Max slipped past him and jumped down onto the tracks.

Thomas could hear the sound of rats scurrying out of the way. Pippa was whimpering behind him. He kept one hand on the wall for balance, moving as quickly as he could along the tracks, careful to avoid the third rail, heart leapfrogging in his throat. He felt each passing second as if it had a separate taste and texture, flaking away like snow, dissolving.

Then he saw the glimmer of lights from the subway station ahead of him. Relief broke in his chest. Almost there.

Please, he thought. Please, let us be in time.

They came out of the darkness, panting. They hauled themselves up onto the platform and dashed up the stairs into the street. They sprinted to the museum, shoving past people on the street. Thomas scanned every face, half expecting Rattigan to jump out at them, leering, triumphant. But he saw nothing but strangers: men nose-deep in newspapers, children hurrying to school, women calling to one another from windows and stoops. Sam plunged past a display of vegetables at the corner of Eighth Avenue, upsetting an entire tray of bruised tomatoes to roll into the street.

“Hooligans!” the vendor cried, waving his fist.

“Sorry!” Sam called back.

Finally, they arrived. They burst through the front doors and skidded through the empty lobby. It was quiet and very still. Thomas thought his heart might rocket out of his chest. Through the Hall of Worldwide Wonders; up the spiral staircase to the third floor.

They burst through Mr. Dumfrey’s office door together.

“Children!” Mr. Dumfrey was sitting behind his desk, wearing his scarlet robe and a pair of felt slippers, sipping from a steaming cup of hot chocolate. “Where have you been? I’m afraid we’ve had a bit of a slow start this morning. But what a glorious triumph last night was! What a brilliant success!”

“You . . . you’re all right,” Thomas panted out.

“Of course I’m all right.” Mr. Dumfrey removed his glasses and squinted at Thomas. “What’s the matter with you? What’s the matter with all of you? You look paler than Quinn and Caroline.”

“Mr. Dumfrey, you’re in danger,” Max said between gasps.

Mr. Dumfrey frowned. “Don’t tell me you got into the champagne last night,” he said sternly. “You’re far too young. Much better to start on beer. Kidding!”

“It’s true, Mr. Dumfrey,” Thomas said. But even as the words escaped his mouth, he wasn’t sure. Dumfrey’s office looked the same as it always did—no booby traps or trip wires or knives dangling from the ceiling, just stacks everywhere, piles of boxes and papers and Cornelius hopping around his cage, squawking. Could Rattigan have been bluffing?

Then Pippa stiffened. “Your pocket,” she said. “There’s a note in your pocket. For a delivery.”

Mr. Dumfrey sighed and returned his glasses to his nose. “What have I told you a thousand times, Pippa? It’s rude to read people’s pockets without their permission.”

“Did someone send you a package?” Thomas asked.

“Not ten minutes ago,” Mr. Dumfrey said. He withdrew the note from his pocket and smoothed it on the desk, then read aloud: “‘Dear Mr. Dumfrey. I believe this belongs to you. Sorry for any trouble I’ve caused.’ Wonderful, isn’t it? The thief must have had a change of heart. And now the shrunken head has returned!”

“Who brought you the head?” Thomas said. He was gripped by certainty now: Rattigan. Had to be.

Mr. Dumfrey continued to stare as if the children had lost their minds. “Nobody brought it to me,” he said, frowning. “The doorbell rang. When I opened the door, I found the note and the package on the doorstep. I imagine the thief was ashamed to show his face.”

“Oh no,” Max muttered.

“Mr. Dumfrey.” Thomas was struggling to breathe. “Where’s the head now?”

“By Barnum’s britches, what’s going on?” Dumfrey straightened up a little. “You’re acting very strange.”

“The head, Mr. Dumfrey!” Thomas’s throat was tight with panic. “Where is it?”

“Right over there.” Mr. Dumfrey pointed to a small cardboard box, open to reveal mounds of tissue paper. “And looking not much worse for the wear, I’m happy to say. Now will someone explain to me—”

But Thomas was no longer listening. He dove toward the box and hefted up the head from its bed of tissue paper. It looked roughly the same. But it was too heavy. Much too heavy.

And Thomas could hear a faint mechanical ticking, coming from somewhere directly behind the head’s glass eyes, getting faster and faster.

The head began to vibrate in Thomas’s hands. Dumfrey sprang out of his chair, overturning it. Suddenly, everyone was shouting.

“Max!” Thomas shouted. “The window.”

She understood him at once. There was a small window fitted high in the wall. As Thomas vaulted up Dumfrey’s bookshelves, and hurtled himself into the air, Max grabbed a fountain pen from Dumfrey’s desk and shot it straight through the window. Glass shattered outward, and Thomas felt a blast of wind just as he released the head, hurling it as far as he could.

There was a thunderous blast as the head exploded in midair, bright as a second sun. The walls shook. Sam and Max ducked, and Dumfrey pulled Pippa out of the way as a massive stone bust of Benjamin Franklin fell from its pedestal, shattering in heavy pieces directly where she had been standing.

Thomas landed badly, grunting, and an avalanche of books thunked onto his head. The room smelled like smoke and singed paper.

For a moment, Thomas thought he might be dead.

“Thomas!” Then Dumfrey’s face appeared above him. As Dumfrey unearthed Thomas from the mountain of books encasing him, more faces came into view: Pippa and Max and Sam, all of them wearing identical expressions of concern.

“I’m all right.” Thomas sat up, groaning. “At least, I think I am.”

“You did it, Thomas,” Sam said. A slow grin spread over his face. “You saved Mr. Dumfrey’s life.”

Thomas tried to smile, and winced. He’d accidentally clunked his jaw when he fell. “We did it,” he corrected. And, extending a hand to Pippa, he allowed himself to be helped to his feet.