Book 6, a continuation of the Tom Gray series.
When the body of an MI5 operative is found floating in the Thames, police frogmen find a significant clue nearby: Nikolai Sereyev, an MI5 informer and mid-level player in a Russian criminal organisation. Both men have been brutally murdered.
Andrew Harvey is tasked with finding his colleague's killer, and quickly uncovers a plot to assassinate a visiting dignitary on British soil. No sooner has he scraped the surface of the case than the tables are turned and he becomes a pawn in a game of international brinkmanship that leads all the way to the Kremlin.
Harvey's girlfriend, Sarah, also a secret service operative, is hot on his trail, but when she too becomes compromised, security chief Veronica Ellis knows there is only one man she can turn to. He's a loose cannon, but she needs his help to rescue her agent and prevent a full-blown international incident.
The trouble is, Tom Gray has gone to ground. Finding him is just the beginning.