December, 1916. Inspector Sam Blackstone is suspicious when he is called to the office of Superintendent Brigham - the new head of special branch. A German traitor wants £25,000 in exchange for intelligence on U-boat locations, and 'Max' has specifically asked for Blackstone to be the one to deliver the money - alone. Blackstone, and his sergeant, Archie Patterson, are certain there's more to it, but Blackstone can't turn down the job. But that is little comfort when, as suspected, it all goes wrong, and Blackstone finds himself on the run from his own colleagues, with Patterson languishing in a cell. A chance meeting with a Russian former colleague seems to provide a lifeline, but his help comes at a cost...