Ali "The Cat" Armeni is a terrorist with the United Pakistan Army Commando.
Scott Belgrade is a high school teacher.
This series is about the ongoing intentions of the former to attack the U.S. from inside and the equal determination of the latter, a man in the wrong place at the right time, to stop him.
UPAC is a relatively small but well funded Islamic terrorist organization based in, obviously, Pakistan with the intended goal of striking at the heart of infidel nations, first and foremost being the Great Satan, America. It was lead by Mustaf al Hezeri, a rich and influential man with a particularly virulent nature when needed.
"Ali had grown up in northern Pakistan in an impoverished family. Showing shrewdness, resourcefulness, and intelligence, the eldest son found ways to get money, food, and essential needs for his family. His life changed at the age of thirteen, when he picked the pocket of a UPAC leader, Mustaf al Hezeri. Luckily for Ali, when Mustaf discovered Ali had stolen from him, he recruited Ali rather than tortured him. Mustaf figured anyone who had the balls to pick the pocket of one of the richest and most powerful people in the area might be useful.
"Over the next five years, Ali found himself not training for combat, but rather going to school and then attending a university, studying to become a mathematician. After his years of schooling in Pakistan, Ali got a student visa and enrolled in the teaching program at C.W. Post University in Connecticut. Following graduation, Ali got a job as a math teacher at Middlesex High. Because of the need for math teachers in the United States, he was fast-tracked and became a United States citizen."
Which is where we first encounter him.
Also teaching at the school is Scott Belgrade, a new hirer there as a health teacher, coming to the facility after a run-in that did not end well at his previous school. Belgrade is married to Joy with a four-year old son, Christopher. Belgrade is described as having a "deep baritone voice that didn't quite match his slim, athletic build".
Belgrade is a fairly average guy but possessing a strong sense of duty. When he goes up against someone he had tried to be friends with, Ari the Cat, and the others from UPAC, he finds in himself a lot more resolve than even he knew he had.
Belgrade most certainly did not have any thoughts of becoming a part-time consultant to the F.B.I. but when UPAC attacked the school where he worked and he was able to help thwart the plot, he became the man who knew the terrorist and was in the best position to advise about him.
As Ali the Cat moves on to other targets, the F.B.I. brings Belgrade along with them to stop him.