The Cryptic Cold Case

Can Dora and Rex solve this cold case without ending up in hot water?
London, 1923. In a Mayfair dining room, Theodora Laurent and Lord Rex Bankes-Fernsby make an incredible discovery. Rex's sister Caledonia is a naturally gifted codebreaker.
Upon hearing this news, Lord Audley, London's spy master, asks for their help with a most unusual mission.
Twenty-five years earlier, his wife died in a terrible accident. Now, he has uncovered evidence that her death may have been murder. The answer to what happened to her lies in the coded messages he discovered in her old things.
With Caledonia's help, Dora and Rex delve into the mysterious case of Lady Audley. Each decoded message reveals a tantalising clue. But, in the excitement of the chase, they forget one fact.
They aren't the only ones who know what those messages say.
The closer they get to the truth, the harder the killer will work to keep their misdeeds buried in the past.