The series is composed by three games:
-) Impossible Mission
-) Impossible Mission 2
-) Impossible Mission 2025
In the first two (developed for Commodore 64 and various other platforms), you are secret agent 4125 and must foil the plans of the evil genius Elvin Atombender by infiltrating his stronghold (well guarded by several robots) and gathering the info scattered in several different places to unlock the code needed to access the final room (and stop Atombender).
You have no weapons (albeit in the second title you can get a very limited number of mines and bombs): dexterity and correct timing are needed to avoid the robots while searching all sorts of furniture for the much needed info (maps in the first title, code numbers allowing in triplets to recover a fragment of the musical password in the second title) and the occasional bonus (e.g. temporarily deactivates robots).
The stronghold is divided into several rooms connected by corridors and elevators (randomly generated each game to increase replay value and avoid pattern memorization) and each room occupies one screen, giving a sense of many mini-platform games linked together by the "gathering info" goal.
The third title (only available for Amiga) is more loosely connected with the others. You may play as one of three characters (none being Agent 4125) and the action takes place in a more classic platform style, with big linear levels spanning through several screens. While elements like the robots to avoid were still present, the overall feeling is less unique than that offered by the two predecessors.
In all the three games, you have no "lives", but a certain amount of real time to complete the mission (8 hours in the original) and when you "die" (killed by the robots or falling into a bottomless pit) you get some time deduced from the amount left (10 minutes in the original). When the countdown reaches zero, the game ends in defeat.