Berserker, the most successful assassin the CIA ever had. Feared by everybody, even those within the agency. But it was an ill-deserved moniker for Mason Crest. Mason was broken, the result of three years in a private Russian prison at the mercy of the Wagner private military force. After being exchanged he lived in absolute pain, existing by means of a ketamine mix made by a drug dealer, and a handful of pain medication. He skirted the boundaries of addiction, but knew that ultimately this was no life for him.

Then he met Caitlin, an operative on the cusp of being relegated to a desk job, as she did not possess the mental requirements to be an effective field operative. But she possessed compassion and empathy, which was what they needed to reach Mason. Nobody could predict the effect she would have on Mason. Together they find a new purpose as a highly effective Hunter/Killer team, all whilst Mason deals with surgeries to repair the damage done to him in prison.

They also navigate his mental healing, Mason finding himself in a very unfamiliar role as the Agency shows their deep admiration for his unorthodox methods.