Tanya and Junior are freelance agents for the American government.
That is really not true but when you read this next part, you will see why I sort of lumped them into the "freelance" category.
The two people are twins, seventeen years of age when we first meet them. As Junior tells us right away, "Tanya. We speak six languages. We've lived in Italy, Pakistan, Germany, Mexico, China, Russia, and most recently, we now live in Greenwich, Connecticut. Every time we relocate, Dad changes our names. Tanya and I have never met our mother, and our dad refuses to answer questions about her, but what Tanya and I have lacked in identity has been compensated by a posh lifestyle. We're not exactly sure how our father affords this posh lifestyle, but it's all we've ever known."
We learn soon after (well, apart from being told in a blurb about the series) Dad is an assassin for the CIA; a very successful one who is paid very well for his many completed missions. Now, neither Tanya nor Junior know this; Tanya does not care enough to wonder and Junior lets his imagination go crazy with speculation. When they do learn the truth they also learn that for reasons I could not figure out, once they found out, the powers that be in the CIA knew right away and a kill order was put out on them (why? no clue).
Once they learned the truth, they also realized why they had been trained all their lives in the sorts of things that would help them survive when really bad things start to happen to them - which they do. They both had been taught martial arts (Tanya is better) and computers (Junior there) and marksmanship and how to blend in and a plethora of spycraft. All these will be needed when the Agency comes a-killing and they go on the run.
After that, well, things get screwy really fast and as time goes by, they will get drug cartels and New York mobsters and would-be government overthrowers and much more all after them but luckily they manage to save the life of someone very powerful and he is most appreciative of that.