Maggie and Jennifer Arnold are students.
They are not actually, formally, or officially spies in any actual, formal, or official organization. They are, as stated, students because they are both 13 years old. We know they are the same age because they are twins.
Having stated that neither of them are spies, it is important that I let you know that in the case of the strange name for this collection of three recorded adventures, Jennifer is indeed "The Spy Who Wasn't There".
That part is all Jennifer's fault; she is the one who came up with the great idea of "borrowing" the book of magic from the University library and then start messing with the spells inside. Maggie, as always, it seems, recognized this as another instance of "here we go again ... Trouble!" Yet another result of being the elder of the two, by 8 minutes, and thus the more responsible one. She was always having to deal with the situations that Jennifer gets them into. This time was way different, though, because this time the spell to turn a person invisible actually worked, which is sort of cool, but there did not seem to be one to turn that person visible again. See? Trouble!
Of course, there does turn out to be a way to reverse the process and make Jennifer visible again - reading the spell, written in Latin, backwards. That's cool, too, except later Jen finds out that at stressful times, she kinda sorta fades away all without her having said anything. That is definitely not cool.
Moving past the whole young teen invisible girl situation, which is pretty neat and could definitely be an asset when dealing with spy stuff, the matter of how the twins actually find themselves involved with foreign agents and dangerous stuff like people getting dead around them - that all falls on the shoulder of their Grand. Grandmother Arnold, that is, their grandparent on their father's side.
Globetrotting Granny is how their father described his mother to them a couple years before - back when they were around 10 and just before the divorce and Dad, a commercial pilot, walked out of their lives. That was also about the last time they got together with Grand. Until now. Grand was a constant traveler and do-er of really cool stuff like white water rafting and jungle exploring and stuff and so on. And when she invited the girls on a trip, this time they got their mother to finally agree to let them go.
And that is how the two girls start going out to exotic places. As for getting involved in spies and intrigue and a whole lot of danger, well that is kinda their own fault. Well, mostly Jennifer. Okay, maybe the CIA, too.