When it comes to a woman's neckline, there is low-cut, and then there is Absolute Cleavage. This is when a dress or top is specifically designed to show the entirety of the wearer's cleavage and the top bit of her stomach. Or the bottom bit, if they really want to court tabloid immortality.
A catsuit is frequently one zipper away from Absolute Cleavage. This is possibly justified in that, if you're dumb enough to wear a tight-fitting catsuit on a spy mission in the first place, unzipping the front will give you a lot more mobility in the arms and shoulders. There may be a bra underneath (and if so, it's probably red or black), but generally there isn't one.