CIA headquarters in Langley, Virginia, receives a cryptic message from an agent operating deep undercover in Russia: "He has betrayed us and wants to go to war." Unable to make contact, the Director of Operations is forced to turn to one of his most deadly field officers—Will Cochrane. His mission: infiltrate the remote submarine base in eastern Russia, locate the agent operating under the codename Svelte, and decode his message—or die trying.

It's a near-impossible task—even for a man who carries the codename Spartan, a title given to the most effective Western intelligence officers. Will successfully locates the base but finds Svelte near death, his last words a final clue: Only Sentinel can stop him. Now it's up to Will to uncover the true meaning of Svelte's message . . . and discover the identity of the legendary operative known only as "Sentinel."