I'm talking about the original here, mind you, not later updates.
TS was, to my knowledge, the very first espionage RPG. I'm probably wrong here, but I was a TSR freak when I was a kid, and it was TSR's espionage RPG. Horrible, horrible, horrible. It had three (three!) character classes, Investigator, Assassin, and, um, something else. Kidnapper or something. In any case, you did not earn ANY experience whatsoever for doing something that was in the forte of one of the other two classes. If you were an Investigator and you killed the arch-nemesis of your group, it didn't matter. Zero x.p.
Mind you, this was something of a radical idea back in these days, that you didn't get XP for killing everything in sight, but it was a lousy mechanic and it didn't work right. Far better to go with what they (TSR) later did with Star Frontiers, which was to go with a flat amount per session, modified by character actions slightly.
There was also a startling lack of direction to the game; it seemed that while it had a lot of information, it also lacked a lot of basic story ideas, plot elements, and the other things that made a spy movie different from an episode of the A-Team, which is what most of my Top Secret adventures seemed to end up like. (I was only thirteen, of course, but...)
The only adventure I ever bought was something like "Operation: Sprechenhaltestelle", no, I am not making that up, and it was horrible. Damned if I remember what it was about, other than a big complex underground tunnel system. Which made it, of course, into an AD&D adventure. The module didn't have any real motivation or plot to it, but rather was a setting for possible adventures. But we were thirteen-year-old boys, of course, and didn't realize that. So we wandered around, blowing things up.
And getting really pissed off when only the Assassin got any xp for it.