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. Characters gained one attribute point for every hundred experience points in addition to going up in level. The levels had relatively limited in-game effects (most significantly, gained experience points were divided 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 only in-game effect of a character's bureau is that he earns a 100-point experience bonus for mission objectives which fall within that bureau. An appendix in the rule book lists dozens of historical and fictional espionage organizations which could serve as employers or adversaries for missions.
In 1987, TSR published Top Secret/S.I. ("Special Intelligence"), a revised edition designed by Douglas Niles. S.I. introduced a more structured gaming environment in which players worked as agents for secret intelligence agency ORION against its evil adversary, WEB. Later source books in the product line introduce both supernatural (Agent 13) and futuristic (F.R.E.E.Lancers) adventure settings. These settings introduced several recurring characters such as Sebastian Cord and Agent 13.
Top Secret/S.I. uses a fast, simple combat system based on percentages. With as little as a single die roll, a player can know not only if a character was hit, but what part of the body was hit and the extent of damage.
Based on a character's stats, skills, bonuses and penalties, the gamemaster gives that character a certain percentage chance of hitting a given target. The player then rolls a percentile die; a result that is equal to or lower than the to-hit percentage succeeds. The hit location is determined by the 'ones' digit of the same roll, and hand-to-hand combat damage is determined by the 'tens' digit. Weapon damage ignores the 'tens' of the first roll, and requires a second roll based on the weapon's characteristics.