The premise for SR is simple: There are three gangs. You are recruited by a fourth to annihilate the other three. Kill the police. Do non-linear missions, win.
The premise for SR2 is simple: There are four gangs. You are the leader of one. Annihilate the other three. Kill the police. Do non-linear missions, win.
The premise for SR:TT is simple: There are two gangs. You are the leader of one. The other one is divided into...three sets. Annihilate the three sets. Kill the police. Do semi-linear missions, win.
Very similar games, all fun as sh!t.