Mission Critical by Mark Greaney
From Mark Greaney, the New York Times bestselling author of Gunmetal Gray and a co-author of Tom Clancy's Jack Ryan novels, comes a high-stakes thriller featuring the world's most dangerous assassin: the Gray Man.
Court Gentry's flight on a CIA transport plane is interrupted when a security team brings a hooded man aboard. They want to kick Gentry off the flight but are overruled by CIA headquarters. The mystery man is being transported to England where a joint CIA/MI6 team will interrogate him about a mole in Langley.
When they land in an isolated airbase in the UK, they are attacked by a hostile force and the prisoner is kidnapped. Only Gentry escapes. His handlers send him after the attackers, but what can one operative do against a trained team of assassins? A lot, when that operative is the Gray Man.