Matt Drake is an agent with the DIA.
When we first meet this highly experienced operative, it is at the airport in his hometown of Austin, Texas. He is looking at a beautiful little girl named Abir and trying not to see her. Abir and her parents had died three months earlier and Drake knows it was his fault. He had broken a promise and they paid the ultimate price. He had very good reasons and he did not act maliciously but they died nevertheless and now he is suffering from PTSD and survivor's guilt.
He will have to deal with both as his country will call on him again to go into action.
This despite the fact that when we meet him, he was no longer in the employ of the DIA. He had been, obviously, for over half a decade but since the trouble in Syria that left his friends dead and he with the tremors in his hands at times, he was no longer working for them, or more specifically, for Branch Chief James Glass. Glass, however, was not one to take no for an answer and when he needs Drake to come back into the fold, he is determined to get him.
Hence Drake's need to lose the shakes and the seeing of ghosts and the guilt. There were still very bad men out in the real world that Drake would be asked to eliminate.
One very interesting and enjoyable (well, to a point) aspect to these stories is Drake's on-again-off-again relationship with Laila, his wife. I say to a point because this is a relationship that needs considerable work to make it function and Drake's PTSD coupled with his boss's determination to pull him away from his home in Texas and into trouble-spots around the world do not make that likely.