Danielle 'Dani' Reynolds is a famous fashion photographer. Mac Harper is a heavy construction equipment operator.
Those are hardly two lines of work that would lead people into working with an American intelligence organization.
Their entry into the cloak and dagger world comes in an interesting way.
Reynold's husband, Mark, was not a 'gentle, soft-spoken unassuming' man with "an ordinary job with the diplomatic corps" as she had always thought. She was on a normal photo session when she receives a phone call from a woman in a Central American country informing her that Mark had been killed. She also let Reynolds know that Mark had been for quite some time an 'outrider', a term they used for a person with 'no official cover' for the government.
Reynolds' next action was to head to the man she knew to be Mark's boss, Henry Towler. Here it gets a bit mysterious, rather appropriate for this line of work. Exactly where does she go? All we see is a tall building with an address on 'Federal Plaza', almost certainly in New York. Does Towler work for the State Department as Mark's 'diplomatic' cover would suggest or somewhere else? In truth, it really does not matter as Towler will recommend strongly that Reynolds do nothing. He will come to change his mind.
Reynolds is determined to head to site of her husband's murder. She is smart enough to know she will need help. Enter Mac Harper. Harper when we first meet him is recently out of the military operating a 'catepillar', hence my putting his job down as working construction. Reynolds' knowledge of him comes first from a publicity poster Harper was pressured to do as a recruitment effort in his final days with the Army, followed by making a few calls to learn more about him.
Her unofficial dossier on him says that this former Green Beret had "special training in martial arts, advanced skills in the use of chemical interrogations, marksmanship citations and advanced field experience in weapons, multilingual (French, German, Spanish, and Chinese), a Distinguised Service Cross for bravery and others." At her request he comes to New York for a meeting (more out of curiosity than anything else, it seems to me).
That is where Renolds tells Harper, "I would like you to help me find the people who murdered my husband and why, and then I would like you to help me punish them."
Towler was initially against them getting involved. After they showed how well they could handle trouble, he, being a sharp handler, started to use them.
One third of the way through the series of adventures that Reynolds' will take on for Towler, Harper is replaced by Jack Striker, an operative already working for Towler. Harper was born in Kansas, according to him, but at age 13 after the death of his parents, he was sent to live with a relative who ran a sheep ranch in Australia.
When Dani talked of exceeding their orders from Henry and trying to snag a killer, Mac agrees with, "I like your style. I spent a lot of time in the stockade with people who think like you."