The Lazarus Protocol

A retired MI6 cryptographer is murdered in a locked Highland cottage, leaving only a Cold War cipher no one has used in thirty years. Sidelined field officer Daniel Mercer is pulled back in - because his late father helped build the code's secret. The trail runs through Prague, Vienna, Istanbul and a buried Soviet bunker in Estonia, chasing a mysterious figure called the Curator, who's resurrecting a Cold War contingency that hides sleeper identities inside governments, banks and crime syndicates worldwide.
But Mercer soon learns Lazarus was never one protocol - it was two. And the second one might force him to erase the only proof that someone he loves has been living a lie for decades.