The original version of Top Secret was designed by Merle M. Rasmussen, and allows players and gamemasters to build their own espionage story settings. The original boxed set of the game included a 64-page rule book and a sample adventure, "Operation: Sprechenhaltestelle". The game was developed over a period of two years by Rasmussen and TSR editor Allen Hammack. As part of the playtesting for the game, a note written on TSR stationery about a fictitious assassination plot brought the FBI to the offices of TSR Hobbies.
The Top Secret game is based exclusively on 10-sided dice. All character attributes and other statistics are percentiles; some scores are rolled, and some are derived from combinations of two or more other scores. Top Secret also features Areas of Knowledge, which function similarly to skills in more modern RPGs. Characters gain experience points and progress upward in level. The levels had relatively limited in-game effects (most significantly, gained experience points were divided by the character's level but the base mission pay was multiplied by the character's level).
Top Secret characters are employed in specific bureaus — Assassination (Killing), Confiscation (Theft), or Investigation (Research) — all in the structure of an unspecified espionage agency. Despite a character's primary vocation, he may be called on to perform any type of mission. The in-game effect of a character's bureau was a 100-point experience bonus for mission objectives which fall within that bureau as well as bonus mission pay for those actions specific to the chosen bureau. An appendix in the rule book lists dozens of historical and fictional espionage organizations which could serve as employers or adversaries for missions.
An expansion to the game, The Top Secret Companion introduced enhancements to many game components. It included additional character classes and missions, as well as new Areas of Knowledge and abilities. A revised combat system was introduced that sped up and provided more variety to combat results. New equipment and weapons were introduced as well.
Source: Wikipedia, "Top Secret (original edition)", available under the CC-BY-SA License.