Lizzie slips off the departing ship at the last possible moment, choosing to remain in England and find her mother.

Jakob is ordered by the Colonel to collect Lizzie from a secret London postal address.

Lizzie finds a small locked leather diary hidden beneath a loose floorboard in Willa's closet.

Lizzie signs the Official Secrets Act at Bletchley Park and is given a new postal address.

Jakob and Lizzie move into The Shoulder of Mutton Inn, where they meet the owner's son Colin.

Colin easily springs the lock on Willa's diary, revealing the entire contents are written in code.

Lizzie is assigned as a messenger at the Park and meets Marion, a young Austrian Jewish refugee.

MI5 agent Jarvis confronts Jakob by the lake and suggests Willa may have defected to the Nazis.

Lizzie and Colin travel to London and meet Olivia, who confirms Willa vanished on the day of the Warsaw evacuation.

Marion takes Lizzie's place to deceive Fleetwood at the embassy party, allowing Lizzie to remain in England.

The codebreakers exploit operator mistakes to crack the Enigma code and intercept Nazi invasion plans.

The Colonel meets Jakob and Lizzie in a London hotel room and reveals Willa is alive and a British MI6 spy.

Willa disguises herself as a tea lady, briefly enters the room, then slips away with the diary.

German planes bomb London while Jakob and Lizzie take shelter in a bunker beneath the burning city.

The Blitz rages on but Britain holds firm, and everyone gathers at The Mutton for a toast.

The story begins as Lizzie defies orders and stays in England, determined to find her supposedly dead mother Willa. Jakob collects her and brings her to the top-secret codebreaking facility at Bletchley Park, where both siblings become entangled in the war effort. While Lizzie works as a messenger and builds new friendships, MI5 agent Jarvis circles their family with suspicions about Willa's loyalties. The siblings uncover hidden coded messages and piece together the truth of their mother's disappearance, racing toward a secret rendezvous in London. The Colonel finally reveals Willa is alive and a heroic British spy, though she can only visit her children in a brief, dangerous disguise. The story ends with the Blitz underway and London burning, but with Britain's spirit and the Bletchley team's resolve unbroken.

The core mystery and wartime mission unfold.

Jakob races to catch a departing train after the Colonel orders him to retrieve his runaway sister.

Jakob and Lizzie visit the Mayfair family flat and meet their imposing grandmother, Gran.

Lizzie fails to pick the diary lock but finds a memo revealing Willa's Embassy posting and the name Olivia McQuatters.

Jakob teaches new recruits Beryl and John about the Enigma machine, explaining its many layers of complexity.

Beryl quips that testing every possible Enigma combination would take longer than the age of the universe.

The crowd at The Mutton gathers around the radio to hear one of Churchill's famous wartime speeches.

Jakob and Nigel spy from a tree as Alan Turing secretly buries silver ingots in the ground.

Lizzie and Colin hide inside a closet as Fleetwood enters and searches the family flat in London.

Colin spots a family photograph and reveals he has seen the woman pictured before.

Ambassador Kennedy tells Lizzie privately that the Warsaw assignment was entirely Willa's own idea.

Lizzie escapes Fleetwood at the embassy party by hiding in a dumbwaiter and fleeing with Marion and Denny.

Jarvis searches Jakob's room and threatens to have him removed from Bletchley and sent to the front lines.

Lizzie demonstrates thorough knowledge of the Park's operations, and the Colonel assigns her directly to his office.

Colin confesses romantic feelings for Lizzie and asks her to promise not to go to London alone.

A German bomber drops two bombs on Bletchley Park but causes no serious damage to the operation.

As Jakob and Lizzie navigate wartime Bletchley, secondary moments reveal the texture of their experiences and the people around them. Jakob trains a new team of codebreakers while Lizzie pursues clues through London and forms key alliances with Marion and Colin. A tense visit to the family flat leads to a near-discovery by Fleetwood and a crucial revelation from a family photograph. Lizzie's growing boldness takes her to an American Embassy party, where she outwits Fleetwood and pries vital information from the ambassador. Jarvis's escalating threats put Jakob's position at Bletchley at serious risk, while Lizzie's sharp observations earn her an unexpected promotion. The section closes with Bletchley itself coming under attack but surviving intact, and the community finding solidarity despite the ongoing war.

The friendships, investigations, and close calls behind the main story.

Lizzie deliberately exhausts Fleetwood with a long walk before abandoning him and secretly fleeing the ship.

Jakob tells Lizzie that Willa is probably dead and that perhaps it is better that way, shocking her.

Jakob recalls how Gordon Welchman tested his lateral thinking with cryptic questions and recruited him into codebreaking.

The Colonel questions Lizzie about what she knows of Bletchley Park but ultimately agrees she can stay.

Jarvis first identifies himself to Jakob as an MI5 agent and warns that Willa may be a traitor.

Lizzie discovers that Willa's newly unlocked diary is written entirely in an unreadable code.

Jakob shows Lizzie a newspaper clipping from Willa containing a secret pinprick code reading 'Your father would be proud.'

Jarvis confronts Jakob with news of an American Embassy spy who has implicated Willa as an accomplice.

Jakob and Lizzie paddle to the middle of the lake, and Lizzie finally reveals Willa's hidden diary to her brother.

Alan Turing advises Lizzie to consider shared family memories as a key to decoding the word 'goulash.'

Gordon Welchman reveals that a woman matching Willa's description helped evacuate Polish mathematicians on a train from Poland.

Jakob and Lizzie decode three separate clues from Willa pointing to a meeting date, the diary, and a London address.

Lizzie secretly departs for London carrying Willa's diary and evades her MI5 tail in the city streets.

Marion takes Lizzie's place on the plane to America, allowing Lizzie to secretly return to Jakob.

Jarvis withdraws his investigation after being told Willa has died, and the group finds comfort in one another.

The story's turning points begin with Lizzie's bold choice to stay in England and Jakob's emotional distance as he wrestles with grief and secrecy he cannot explain. Key confrontations and coded messages from Willa gradually reveal that she may not be dead at all, forcing the siblings to question everything they believed. Lizzie and Jakob slowly lower their guards with one another, sharing clues from a rowboat on the lake and receiving vital help from Alan Turing and Gordon Welchman. Together they decode Willa's messages and race to London, navigating MI5 surveillance along the way. The Colonel's revelation about Willa's true role as a British spy resolves the central mystery, though the family cannot reunite until the war ends. The story closes with the immediate invasion threat averted and the characters finding hard-won peace together, even as the broader war rages on.