Agnes 'Aggie' Westfeld is an agent with the CIA.
Agent X, aka Sam, is also one, sort of.
Westfeld is quite junior in her new line of work being in her early twenties and only with the Agency, after standard training, for a couple of months. Imagine then her surprise and considerable unease when she is summoned to the top floor by the Deputy Director. Increase it some when that man, Danton, tells her she was not his first choice for an upcoming field assignment. This would let her know two facts: first, she was being sent on her first mission, and second, someone higher than the Deputy chose her. Why, she had no clue.
Which brings us to Agent X. He is the higher authority, again sort of, that wanted Westfeld in a more active role in the Agency.
He is, no spoiler here if you take a look at the cover of either recorded adventure, an alien, as in REALLY alien. "Man-like, but slender and pale-skinned beyond belief, with a large bulbous head and eyes...oh, such eyes! Large almond-shaped eyes, black and sparkling and not at all human-like, a narrow chin, a tiny mouth and nostrils without a proper nose". Westfeld's reaction to meeting him was, "Oh my!" followed moments later with "Ohhhh...myyyyy!"
Sam, which Westfeld came to call him because of his total devotion to detective noir, complete with fedora and trenchcoat - hence Sam Spade, was one of the crew of an alien craft which crashed on Earth many years ago - the sole survivor of that bad landing. Since then he has been assisting the American government on various assignments and had decided a human partner was in order. Why a junior operative like Westfeld? Well, it's a bit complicated.
Still, there we are with Westfeld teamed with Sam and popping around the world with him on scary, exciting, dangerous adventures. And when I say 'popping', I mean it since Sam can teleport. And read minds. And move objects with his thoughts. And other cool stuff.