What is the plot of Crackdown 2 and is it worth buying

Crackdown 2 is a game about you protecting your sector from cell and zombies at night
while you kill things, you get orbs from guns, agility, driving skills, strength, and grenades. you can splatter your enemies with your car and earn new ones with the more orbs you get. same things with the guns, and grenades.
I'd say it's definitely worth buying.