the alternity RPG was TSR’s attempt at a sci-fi/ modern game. the company already had the best selling fantasy RPG of all time and now wanted to cash in on the other genres. a decade later and WOTC who bought TSR would use many of the same concepts and fluff for their D20 modern game mainly the future supplement.
the core mechanic was in some ways a reversed D20 system where you rolled your D20 and added or subtracted modifiers, but you wanted to roll low so a 1 was awesome and a 20 was super bad. the mechanic was a bit more complex that that though as the D20 roll was accompanied by a situational die or dice. you added dice to make the roll harder and subtracted dice to make the roll easier for example for an average shot with a rifle you would roll 1D20+1D4 and tried to get as low as you could.
the game also had ranks of success such as marginal, good, and amazing. to find out how well you did you took your skill and divided by two for good, then divided it a gain by two for amazing. note those are the names I recall they may be different it has been some time sense I read the book and I do not have it in front of me to check.
the core books included rules for cybernetics, mutations and psionics as well as magic called here FX in the Game masters guide. the rules are flexible enough to run a game in any era of human life. though it emphasized contemporary conspiracy, near future cyberpunk and far future space opera.
i personally love this game, though i can see the wanting to roll low would be off putting to some people. the rules did have the old 80’s early 90’s charm and were far from stream lined, but those are small issues compared to the glory of alternity. therefore i recommend picking this game up if you can find it