As far as I know, Chain Reaction was the first game by Two Hour Wargames.
Its core mechanic is still the core mechanic of all their current games.
When a model moves from out of sight into sight of an enemy, that model takes a reaction test. They are taken on 2d6 agains the REP (reputation, only characteristic in the game, 1-5) of the model (+/- some modifiers), and each dice below the modified REP is a passed die. Sticking with the First Sight Check:
Pass 2
Fire
Pass 1
roll-off to see who fires first
Pass 0
duck out of sight behind cover
A similar table is used when a model received fire and survived, it might return fire, duck down or run away. The duck down and runaway results are the built in 'morale rules' of the game.
When model A fires at model B, but fails to hit, model B takes a reaction test and might fire back at model A, misses too, triggers a received fire check with model A who might fire back and forcing model B to duck down; thus ending the Chain Reaction.
Games of Chain Reaction are quite nice, you get these mini-narratives unfolding; you can also only plan so far, once you ordered your model to take a specific action, you cannot be certain how things unfold due to the reaction tests.
The game is miniature and settings agnostic; the weapon lists provided only place it in 20th century of near future settings. Otherwise you are completely free to play whatever setting you want with these rules.