The Shadow Code by Suzy Henderson

When a WPC cracks a cipher, she ignites a fuse that could bring down a nation.
London, autumn 1940. As the Blitz ravages the city, WPC Ellie Harcourt is called to the scene of a murdered civil servant. Sewn into his coat is a cryptic cipher referencing ‘Kingfisher’, a codename whispered in intelligence circles. Still reeling from the disappearance of her lover, Sinclair, Ellie suspects the cipher points to something far deeper than murder: a conspiracy inside Whitehall.

When MI5 takes over the case, Ellie is paired with officer Jack Stratton – an old acquaintance from society circles with a long memory and a grudge. Their reluctant partnership intensifies when another agent is found dead, another cipher appears, and Ellie’s father – a scientist working on Britain’s top-secret radar programme – vanishes without explanation.

Ellie’s investigative skill and her ability to partially crack the cipher catch MI5’s attention, and she’s invited to consult on the case. As she and Jack grow closer and start to piece together the Kingfisher network, they realise the conspiracy reaches the very heart of Churchill’s inner circle. With traitors hidden in plain sight and time running out, Ellie must risk everything to stop a betrayal that could cost Britain the war.