Janie is seven years old when Japan surrenders, ending World War II. She learns that the Allies defeated two “evil forces” and that the war is over. Her parents’ friends doubt the veracity of this claim, but Janie and her parents, Marjorie and Davis Scott, anticipate improvements to daily life. Janie has an idyllic childhood in southern California. Then, the Korean War begins, and the communist threat grows; her school shows a film giving students instructions about what to do if an atomic bomb falls on the US, and students practice getting under their desks.

When she’s 14, however, Janie begins to feel like she’s being followed. To give herself more confidence, she practices walking like Katharine Hepburn, but one day, she sees a black sedan tailing her as she walks home. Two men in dark suits are inside the car. She manages to lose them and lets herself into her home. That night, the Scotts go to dinner, and Janie’s parents tell her that they’re moving to London because they’ve become targets of the US government for being suspected communists. This horrifies Janie. Marjorie and Davis work in the entertainment industry and fear that they’ll be forced to testify about their friends’ political views. Janie blames her parents for whatever they’ve done to become targets.

Janie is upset about the move. Her parents now use fake names and have secured jobs writing for the BBC. London remains scarred by bombs, and their new landlady, Mrs. Parrish, is displeased about renting an apartment to Americans. They need pennies to put in the gas heater on the wall, so Janie and her father go to the apothecary down the street. Everything about London seems gray, especially compared to Los Angeles. The apothecary is friendly, though, and he portions out two powders for Janie to help with her homesickness.

Janie will attend St. Beden’s grammar school. The secretary introduces her to Sarah Pennington, a “near-perfect specimen” of a girl, who glows with “perfect health” and obvious wealth (20). Sarah escorts Janie to Mr. Danby’s Latin class, explaining that he was a pilot in the Royal Air Force. He was a prisoner of war, and he’s young and handsome. He is a passionate teacher and speaks to Janie like an adult, and she likes him. During lunch, the school has a bomb drill, and the students climb under the tables, but a boy named Benjamin Burrows refuses. He was in London during the Blitz, he says, and lunch tables would do nothing to stop bombs. Janie is scared by what he says but admires his defiant bravery.

Janie is supposed to take the Underground to Riverton Studios to meet her parents, but when she passes the apothecary shop, she sees Benjamin inside. She enters and listens, learning that Benjamin is the apothecary’s son and that he doesn’t want to follow in his father’s professional footsteps. After Benjamin leaves to make a delivery, Janie drops her books off at her family’s apartment, but as she leaves for Riverton, she feels as though she’s being watched. She catches Benjamin following her and confronts him. He says that she interests him, and he rides the Underground with her for a few stops while chatting. Benjamin explains that he doesn’t want to be an apothecary because it’s “boring,” though the Society of Apothecaries pays his school fees. He says he wants to work for the Secret Intelligence Service, something he never told anyone before. Benjamin leaves to make deliveries for his father but tells Janie to meet him at school on Saturday afternoon and asks if she knows how to play chess.

As Janie enters the Sherwood Forest set at the studios, she sees her parents talking to their boss, Olivia Wolff. Janie tells them about meeting Benjamin at school on Saturday, and they tease her about having a “boyfriend.” She meets the actress who plays Maid Marian and watches as her parents and Olivia banter wittily. They treat Janie like an adult, and she isn’t homesick anymore.

Janie meets Benjamin on Saturday. They go to play chess at nearby Hyde Park, and he tells her to watch the man with the wooden leg sitting on the park bench. The man, Leonid Shiskin, is the father of a fellow student, Sergei, whom Janie recalls from Latin class. Shiskin is an accountant at the Soviet embassy, and Benjamin says that he passes secret messages to people through his newspaper. They play chess, though Janie clearly doesn’t know how.

A well-dressed man joins Shiskin on the bench and takes part of Shiskin’s paper. Benjamin and Janie follow him to a nearby hotel. They’re trying to get information about him from the front desk clerk when Sarah Pennington and her mother enter the lobby. The well-dressed man passes by, and the clerk calls him “your excellency” and then tells the children to leave. Later, Janie tells her parents that she’s meeting Benjamin tomorrow for more chess (because Shiskin goes to the park on Sundays too). Janie asks her father for a chess lesson.

When Janie and Benjamin get to the park, Shiskin is already on his bench. Janie’s chess game has improved, and Benjamin is impressed. When Janie looks back, she sees Benjamin’s father sitting beside Shiskin. They’re shocked when Mr. Burrows picks up the newspaper and walks away. They watch as he reads something and then his entire face darkens. He tears up the paper and drops it into a garbage can, and Benjamin retrieves it and pieces it together. It reads, “JIN LO HAS BEEN TAKEN. YOU’LL BE NEXT” (53).

Janie and Benjamin follow him to the apothecary shop, which is locked, but Benjamin opens the door with his key. They find his father burning papers in the back room, and Benjamin asks if he’s a Russian spy. The question horrifies Mr. Burrows, but he has no time to explain because he must hide “the book.” They hear someone rattling the front door, and Mr. Burrows gives Benjamin a large leather-bound text and forces the children into a cellar. He tells the boy to protect the book and keep it safe since it’s been in their family for 700 years.

Benjamin and Janie hear voices above and then an explosion. Men are speaking German. A man with a long scar down one cheek peers into the cellar, but the children hide in the shadows. They hear a police bell outside, and the intruders run off. When it’s silent again, Benjamin and Janie emerge. The shop is a mess, and Mr. Burrows is gone.

Benjamin and Janie take the book (the Pharmacopoeia) to her parents’ apartment. Janie tells them that Benjamin’s father had to visit a sick relative and asks if he can stay there for the night. Lying to them makes Janie sad. Benjamin reports that his mother died in the war. When Davis asks about the Pharmacopoeia, Benjamin asks if he can go read. Later, Benjamin and Janie note a symbol made of circles and triangles on the book’s cover, and Benjamin realizes that he has seen it before.

On Monday, Benjamin gets Janie out of study hall and leads her to the Chelsea Physic Garden. It started in the 17th century as a museum and nursery for medicinal plants and seems almost “magically” green and quiet. They find the book’s symbol on the wrought iron of a locked inner gate.

An elderly man (the gardener) emerges from a small cottage and recognizes Benjamin, opening the gate for them. He asks about the Pharmacopoeia and tells them that Jin Lo is a Chinese chemist. He starts explaining the mysterious symbol, which represents water and the seven operations of alchemy, but he stops when he sees Benjamin’s face. Benjamin says that alchemists are “crackpots,” but the gardener disagrees, pointing out different recipes, like those for transformative elixirs, and highlighting the avian elixir, which changes a human into a bird. Benjamin doesn’t believe it, but the gardener tells him to “allow for the possibilities” (71). He’s shocked that Benjamin’s father hasn’t yet begun to train him, and Benjamin says that he tried.

Janie mentions Shiskin, suggesting that they use the “Smell of Truth” recipe on the Russian. To do this, they must harvest the veritas plant. The gardener points it out and tells them that they must crush the leaves and boil them in water to release the smell. When Shiskin inhales it, he’ll be forced to speak the truth.