A Treacherous Game (Sally Yates, #1)

Detective Chief Inspector Sally Yates is head of an elite task force at New Scotland Yard. When a socialite is brutally murdered in the apartment of a Russian mobster, Yates and her team are brought in to find the killer. It does not take long for her and her partner, Sergeant Andy Dawson, to realize that the murder is the tip of a conspiracy, one dating back to the Industrial Revolution and pre-war Nazi Germany.
In the final days of World War 2, a German U-boat sinks off the coast of Brazil in 1200 feet of water, carrying with it a document in a water-tight safe. In 1945, the technology did not exist to retrieve the safe. In 2022, thanks to deep-sea oil and gas exploration, it does!
The safe and the documents it holds are retrieved. All the parties seeking to recover them, including DCI Yates, converge on Brazil. However, in a twist of events that nobody sees coming, the contents of the safe elude them all. Instead, they turn up in London in the hands of someone ill-equipped to safeguard them. This unexpected twist fools everyone, including Yates and Dawson.
Yates returns from Brazil and must confront the Russian mob and a sinister society, both of whom are searching for the document. The Russians and the society are bad enough, but Yates and Dawson can't trust the British establishment. MI5 and MI6 are looking to retrieve the documents, and Yates believes The Met has also been compromised. Too many people are willing to kill to get their hands on whatever was inside that safe. Yates and Dawson must discover what the documents in the safe are; only then can she expose the Russians, the secret society, and those within her government who are involved in the conspiracy. To achieve this, she must step outside the chain of command, disobey orders, and put her life and that of Sergeant Dawson at risk.