The Ambler Warning, like many of Robert Ludlum's thrillers, gets much of its sense from our knowledge that intelligence agencies cannot be trusted to play fair even by their own. Hal Ambler is held in a facility for insane agents and is himself being drugged to a point where he cannot trust his own crumbling sanity. Even when he escapes, he is unable to be sure of his own past and identity--his very face is not as he remembers, let alone his fishing cabin and the friends of his youth. Something has been made from the wreckage of his mind, and he is not sure whether every step he takes may not be a part of someone else's plan. By contrast, Caston is the sort of intelligence agent who despises the Hal Amblers of this world--he is an accountant who follows the money of assassination and terror round the world while sitting in front of a monitor. Yet he too starts to get a sense that he is being used.