Set in postwar London against the backdrop of the 1951 Festival of Britain, a national exhibition on the South Bank of the Thames designed to celebrate British achievement and lift the country's spirits, the novel follows Harry Flynn, a former journalist haunted by wartime captivity, as he becomes entangled in espionage, murder, and an unlikely family.
In 1949, Flynn sits on a park bench when Gerald Barry, his old editor at the
News Chronicle, recognizes him, having believed Flynn dead. Flynn went to Singapore in 1941 as a foreign correspondent, was captured by the Japanese, and spent years as a prisoner of war on the Burma Railway. After returning to England emaciated and traumatized, he made an ill-considered marriage to Ivy Shears, a war widow with a young daughter, Veronica, and a small white terrier, Mrs. Betty. Both spouses recognized the union as a mistake, and Flynn abandoned the household without explanation. Emotionally shut down since a loveless childhood and further hardened by the war, Flynn had previously contemplated suicide but was saved by the kindness of a stranger. Barry offers him work on the Festival, and Flynn accepts.
The narrative jumps to 1951. Flynn works in a cramped basement office in Savoy Court with Badger (Alistair Badgley), a rotund, affable man secretive about his war service; Evans (Hywel Evans-Jones), a melancholy Welsh former lay preacher who lost his faith during the Arctic convoys; and the Boy (Terence), a fourteen-year-old dogsbody from the BBC. Their nominal boss, Denholm, a former actor, manages them with theatrical diffidence. Giles Dangerfield, a senior Festival official with political ambitions, unsettles Flynn by aggressively asking whether Flynn "served" in the war. Miss Ogilvie (Maud), an elderly, sharp-minded Scottish woman assigned to the office, reveals that Dangerfield married into wealth, worked in the Security Services, and has been placed at the Festival to rehabilitate his reputation. Badger reacts with visible discomfort to Dangerfield's name but refuses to elaborate.
Flynn meets Delphine Dubois, a French woman working in Homes and Gardens, for a drink at the Lamb and Flag pub. Delphine claims someone is watching her and warns that "they" plan to sacrifice her, then abruptly leaves to meet someone she calls "Zheel." Flynn wakes the next morning with a black eye and a total blackout. Delphine does not come to work and is reported missing. Badger quietly offers to help if Flynn is in trouble.
Flynn is assigned to oversee a Festival Cake competition. The judges inexplicably select the worst entry, by Mitzi Blinkhorn, a flamboyant celebrity cooking demonstrator. At dinner at the Blinkhorns' shabby flat, Flynn notices a photograph from a 1936 party hosted by Oswald Mosley, the prewar Fascist leader, with Dangerfield also pictured. Mitzi's daughter Cynthia has extraordinary green eyes, the same jade shade as Dangerfield's. Mitzi's husband, Archie, drunkenly admits that Cynthia is another man's child. Cynthia warns Flynn to be careful. Walking home, Flynn discovers Delphine's silk scarf, stained with what he fears is blood, in his pocket. He buries it in a dustbin, but it later reappears in his desk drawer, planted by someone to incriminate him.
Detectives Inspector Vigo and Constable Tapper question Flynn about Delphine and designate him a material witness. Meanwhile, Flynn encounters Lydia Fairchild, an aristocratic beauty and one of the Festival's orange-sellers, who seduces him. Lydia reveals that Badger is gay, that his pursuit of women is a cover, and that he worked for the Special Operations Executive (SOE), a covert wartime organization that deployed agents into occupied France. She also reveals that Archie is not a real major but was cashiered from the Army for fraud.
Archie pressures Flynn into breaking into a flat in Half Moon Street to retrieve compromising photographs, promising to clear Flynn's name. Flynn enters and finds the photographs, but outside a hired thug named Arthur Stamp attacks him and steals them. Flynn realizes the flat belongs to Delphine. Archie denies killing her but claims she is dead and that Flynn's fingerprints now cover the crime scene.
Ivy reappears with Veronica, now thirteen, and Mrs. Betty, needing Flynn to look after Veronica while she travels to France with her fiancé. Veronica proves bold and resourceful. When Flynn confides his predicament, she insists they confront Tommy Gammage, the barman whom police claim witnessed Flynn fighting with Delphine. Gammage admits he was paid to lie. The next day, Gammage is found dead below his window. Vigo and Tapper arrest Flynn for the murder of Arthur Stamp, found stabbed after their earlier altercation. Maud secures Flynn's release with a fabricated alibi.
Returning to Half Moon Street, Flynn learns from Delphine's neighbor that her gentleman friend had beautiful green eyes and that Delphine called him "Zheel," the French pronunciation of "Giles." Flynn deduces that Delphine was Dangerfield's mistress. Confronted with evidence linking him to the flat, Badger reveals that Delphine was really Colette Paquet, codename "Columbine," an SOE agent he recruited who betrayed an entire Resistance network to German intelligence through her lover: Dangerfield himself, a wartime traitor. Her death was staged after the Liberation of France. When Badger recognized her at Savoy Court, he tracked her to Half Moon Street and found her shot through the heart.
On the Festival's opening day, Flynn spots Arthur Stamp, the man who attacked him, alive and chatting with Lydia. Lydia reveals she works for the Security Service. The plan was to discredit Dangerfield using Delphine as a witness. The night Flynn drank with Delphine, Stamp drugged her drink for extraction, but she left before consuming it and Flynn drank it instead, explaining his blackout. After Badger found the body, the Service removed the corpse, but they do not know who killed Delphine. Lydia clarifies that Maud Ogilvie is actually her superior in the Service.
Before an unauthorized cooking demonstration, Archie cryptically tells Flynn that a jealous female killed Delphine, adding that she should not have taken the gun. An explosion from an illegally rigged gas cylinder kills Mitzi and Dangerfield. Archie vanishes. Weeks later, at the Battersea Pleasure Gardens funfair, a deranged Cynthia confronts Flynn, brandishing the bloodstained scarf and claiming she took it from Delphine's neck. Flynn realizes that Cynthia, Dangerfield's biological daughter, killed Delphine out of jealousy. Flynn grabs Veronica and escapes onto the Big Dipper roller coaster; Cynthia pursues them but falls to her death.
Vigo arrives with news that Ivy and her fiancé have been arrested in France for criminal activity, leaving Veronica in Flynn's care. At a community pageant, with Veronica asleep on his shoulder, Flynn experiences a painful, overwhelming recognition of love. The Festival closes after five months. On its last night, Flynn dances with Maud on the Fairway, the Festival's illuminated central promenade, and resolves to do something useful with his life. Months later, he finds in his wallet the group photograph Maud arranged of the office, all of them caught mid-argument about cheese, looking posterity in the face.