Jakob is walking around the lake. Nigel is with him. The plan he, Beryl, and John discovered seems to be promising, but it takes time.

There’s a commotion in Hut 6, and Jakob rushes over to see what it is. They have cracked the code.

Colin’s mum knocks on the door with some refreshments for Lizzie and Marion. She’s worried, as she hasn’t heard from Simon in a while. They have a plan to keep Fleetwood at bay, feeding him a lot of spirits to keep him distracted. Marion has been quiet for a long time, but she found a string of encoded words in Willa’s diary.

The code breakers have worked out a system to break the Enigma code by finding the setting for each day. They transcribe the messages and pass them to a translator, who then passes them to London Military Command. All the messages are then catalogued by librarians and searched for useful information. Early one morning, Beryl passes a direct order from Hitler to Jakob. The Nazis are preparing for an invasion called Operation Sea Lion.

On the way back from breakfast, Jakob catches a glimpse of Turing’s code-breaking machine. It’s huge. He hears Welchman say that the machine is getting close and that Turing didn’t need to bury that silver. Jakob shadows them and is quickly spotted. It is revealed that Turing invested his life savings in silver ingots and buried them in case of a German takeover. Jakob goes to Nigel and tells him what happened, but Nigel has already moved the silver ingots. Nigel reveals that he hid them, and the only way to find them is with a riddle.

Lizzie has overslept and is late for her shift. Jarvis intercepts her outside the Mutton and tells her he’s from MI5. He asks her about Willa. She tells him about how she requested a transfer to Krakow and that Lizzie thinks she’s still alive. She also lets slip that Jakob met Willa before she left. Jarvis implies that Willa could be a spy, as could Jakob. Lizzie dismisses him. Nigel, in the tree above, has heard everything.

Jakob returns to the Mutton. He finds Jarvis playing a game of chess in the bar with Fleetwood. They finish their game, and Jakob sits down to play Jarvis. Jarvis brings up how Jakob didn’t tell him Willa had visited. He asks if she inquired about Jakob’s work, and Jakob reveals that she did but only in a general way.

Jarvis pesters Jakob with insinuations that his mother is a traitor until Jakob loses his temper. Mrs. Tilbury, the innkeeper, then reveals that Jakob has received a letter. Jarvis tells him to stand up and go with him.

Jarvis makes Jakob take him to his room. Jarvis searches the room while interrogating Jakob. Jarvis says he thinks Willa might be alive and that she could cause havoc if she told the Nazis about Jakob’s work. Jarvis says he’s getting Jakob removed from the Park and sent to the front lines.

Lizzie leaves the Park, and Colin grabs her. He tells her Fleetwood is at the Mutton. Colin then tells Lizzie that Jarvis is with Jakob.

Jakob goes to the Colonel at his desk and tells him about Jarvis and his threats to send Jakob to the front lines. The Colonel scoffs at the idea of putting Jakob into a situation where he could be captured with his knowledge of the Park. Outside, Jakob notices Nigel is watching him now.

Lizzie comes home to find a letter from Fleetwood telling her to pack and that they will be departing that evening. She finds another coded message from Marion. Jakob arrives home. He says he wants Lizzie to be safe and to potentially leave. Lizzie tells him of the Polish names connected to Willa. Jakob recognizes one from something Turing said. Lizzie hands over the letter she took from under the door and says it’s from Willa. Jarvis arrives at the door and tells Jakob to pack his things.

Jarvis refuses to tell Jakob where he’s taking him. Lizzie has to go too. They bid each other farewell. Jakob sits down on the bed and takes out the letter.

Fleetwood and Marion are downstairs. Lizzie makes a big show about being a war orphan, a tactic to put Fleetwood off kilter. Fleetwood has been drinking, but Colin’s not playing his part to get him drunk. Lizzie hears that Colin’s brother was shot down over the English Channel.

Lizzie is loaded into a plane that will head for Ohio, but vows to continue her investigations.

Jakob investigates the newspaper cutting. He heats the paper with a match and realizes there’s an invisible ink code on it that says “BRING WHAT YOU TOOK.” He realizes his mother is alive.

He runs downstairs and finds that Lizzie is already gone. He goes to Colin and borrows his motorbike.

Jakob drives after Lizzie. He catches up with her on the airfield, but she’s already boarded the plane. He screams that she was right, but the plane lifts off. He watches her go and feels a great deal of remorse.

Jakob is in Hut 2 with Beryl and John. They reckon that Operation Sea Lion will be soon. He thinks about the code his mother sent him; he found another on the clipping that simply said “goulash.”

Jakob has been moved to a new billet, isolated from everyone else. There is a new agent stationed to watch him at all times. Someone knocks on the back door.

Lizzie arrives at Jakob’s back door. She switched places with Marion, who is going to America in her stead. Jakob shares that Willa is still alive. He wants to keep the fact that Willa is trying to contact them from Jarvis. He tells her what the code said, and Lizzie thinks about the diary she took from under the floorboards. Mr. Berrycloth arrives home, and Lizzie flees.