“More than just two. Imagine there are thousands, millions, even billions of universes like ours—only in their own dimensions. Benjamin, could you be so kind as to visually demonstrate?”

Benjamin projected a holographic image of Earth before their eyes. Then it duplicated the image. Then again, and again, and again, until the copies filled the entire room.

Matilda could barely wrap her head around the idea. “Exactly like ours?”

Ms. Holiday shook her head. “Not exactly, and that’s where the multiverse gets interesting. Some of these Earths are a lot like ours, while some you wouldn’t even recognize.”

“I have to admit I’m a bit lost,” Agent Brand said.

“Think of it like this,” Ms. Holiday told them. From her handbag she took two candy bars, which she placed in front of Flinch on the desk. “Flinch has two candy bars. He can choose to eat the coconut-peanut bar here or he can choose the one made from nougat and honey. Which one does he choose?”

Flinch looked distressed. It was clear that making this choice was probably the hardest thing he had ever had to do in his short life. His head went back and forth from one treat to the other, like he was watching a tennis match, until he finally snatched the coconut bar. He tore open its packaging and ate it greedily.

“So Flinch made a choice and the rest of his life will move forward according to that choice. But the multiverse allows for other possibilities. If the theory is correct, there is another Flinch, in another universe, in another dimension, where he chose the nougat-and-honey candy bar.”

“Who cares which candy bar he ate?” Matilda said. “What difference will it make?”

“Very little, probably,” Ms. Holiday replied. “But sometimes the decisions are much bigger and have much wider consequences. In the multiverse there’s an Earth where the Germans won World War II. There’s an Earth where Native Americans still control this continent. There’s probably even an Earth where everyone is a pro wrestler.”

“Awesome,” Matilda said.

“Is there an Earth out there where I ate both candy bars?” Flinch asked, eyeing the other treat.

Ms. Holiday giggled. “Yes. There could even be one where you didn’t eat them. Maybe you had carrots and hummus instead.”

“I assure you there is not,” Flinch said, licking his fingers. “There might be a trillion versions of me, but not one of them would pick carrots and hummus over a chocolate bar.”

“There might be a Flinch who is allergic to peanuts and coconut and got very sick from eating the candy bar. There’s one where he is a donkey who likes candy. Another, where he was never born. Still another, where candy was never invented, and so on and so on. All of them exist—they are real—on their own Earths, at least according to the theory. Do you understand?”

“Sure, I get it,” Pufferfish said. “There are a billion different me’s, some good, some bad, some that don’t swell up like a balloon whenever I eat eggs. What does this have to do with Mathlete and her machine?”

Duncan stepped forward. “We can’t be sure until we question her, but I believe she’s using some sort of device that builds a bridge from our world into those alternate Earths.”

“Someone’s been watching too much Star Trek!” Matilda said. “Even if she did build something like that—why? What would she gain from it?”

“We think we know,” Benjamin said. The tiles on the walls flipped over to reveal a massive television screen displaying Gerdie Baker’s face. “Four weeks ago Mathlete visited a dentist. She ordered a set of porcelain veneers for her teeth and had her jaw fractured to correct an unfortunate under-bite. The procedures in total cost nearly thirty-five thousand dollars.”

“So maybe her mother got a good job or won the lottery,” Jackson said.

“According to this report, Gerdie didn’t pay with money. She paid with this.” Brand snapped his fingers and the image changed from sad Gerdie Baker to an ancient treasure chest overflowing with gold coins, pearls, and silver chalices.

Flinch stuffed the other candy bar into his mouth. “Where did she get that?”

“Certainly not from around here. This was found with it,” Benjamin said as one of the coins zoomed into focus. On it was a picture of a strange animal with the head of an owl, the body of a bear, and a long tail like a snake. The creature was wearing a crown. An inscription read, Coin of the Realm. His Royal Highness Doogan the Fifth, King of Zedavia and Surrounding Realms.

“Zedavia?” Matilda asked. “I’ve never heard of the kingdom of Zedavia.”

“That’s because it didn’t exist—at least not on our world. I’ve researched every history book in our database,” Ms. Holiday said. “If it was a real place, I would be able to find it. I may be a spy, but I’m a librarian, too.”

Gerdie’s face came back onscreen, and Brand continued. “A week later, Ms. Baker went to a dermatologist where she was given a laser dermabrasion procedure and a facial and pore treatment that cost nearly two thousand bucks. She ordered a package of ten spray-on tans and a tea bag massage. She paid with this.”

An image of a painting appeared on the screen. It looked a lot like the Mona Lisa.

“She stole the Mona Lisa out of the Louvre?” Matilda asked.

“This isn’t the Mona Lisa. Look closer,” Benjamin chirped as the image zoomed in on the famous painting.

Matilda studied the portrait. It was the same painting she had seen a million times in books. But when she peered closer, she saw something peculiar in the background: silver half-moon–shaped crafts hovering in the sky shooting lasers down on the countryside below.

“An alien invasion!” Matilda said.

“Some idiot painted a copy and added a joke,” Ruby said.

Mr. Brand shook his head. “No, we’ve had art historians study the brushstrokes. This painting was made by Leonardo da Vinci—or at least a Leonardo da Vinci. We found a strand of a brush in the paint and had it tested for age. It dates back to the sixteenth century. The signature is also an exact duplicate.”

“There’s more,” Benjamin said. “The next day, Ms. Baker had a consultation with Dr. Abigail Contessa, a plastic surgeon to the stars in Los Angeles. The day after that she received fifty thousand dollars worth of procedures, including a nose job, collagen injections in her lips, a brow lift, and an ear tuck.”

“You can do that?” Duncan said as he self-consciously tugged on his lobes.

“Let me guess,” Jackson said. “She paid with something that shouldn’t exist?”

Brand nodded and live video of an odd bird appeared on the screen. It had gray feathers, thick yellow talons, and a large beak shaped like the end of a wooden spoon.

“It’s a dodo,” Ms. Holiday said. “Dodos have been extinct for nearly three hundred years.”

“So Gerdie Baker is stealing from alternate worlds to pay for makeovers,” Matilda said. “What do we do? We don’t have jurisdiction over the multiverse.”

“It’s much worse than some interdimensional shoplifting,” Agent Brand said. “There have been what we’ve come to call ‘crossovers.’ Things have been coming into our world—things that should not be here.”

“Like the talking dogs?” Duncan asked.

“Worse,” Brand said.

The screen showed four strange creatures with black tentacles all over their faces. Though shaped like men, each had a wide wound of a mouth filled with sharp, pointy teeth. They were locked in a jail cell, shouting angrily.

“OK,” Jackson said. “I’m officially freaked out.”

“That’s just the beginning,” Brand said.

Matilda’s mind filled with worst-case scenarios. “So we track down Mathlete and arrest her.”

“Not so simple,” Mr. Brand said. “She’s had extensive work done on her face, and her doctors are reluctant to talk to us. Performing plastic surgery on a minor is unethical. Who knows if her face would have changed naturally as she got older? Besides, the doctors only saw her swollen face when she left their offices. Mathlete never came back for her followups.”