Picking up between seasons three and four, the storyline wraps up the Money Train debacle (don't worry - you don't have to know what the hell that means in order to understand the plot) and sees Mackey's Strike Team on the brink of termination. That is, unless they can pull one more high-profile bust. Cue the One-Niners, Byz-Lats and the Armenian mob. For the uninitiated, those are all the factions that will be firing at you, so try and stay alive.
Above: Detective Mackey punishes smut
The Shield is a bit of survival story in and of itself. It saw release through the perils of company acquisitions, and was ultimately resurrected from cancellation by publisher, Aspyr. Thus, the game has spent several years in development, and it shows. Mackey himself looks good, but many of the other characters look positively Muppet-like. And granted, the inner-city LA locales on which the environments are based aren't that pretty to begin with, but they're still pretty bland and blocky.
And surely, some of those years languishing in development hell could've been spent hammering out the glitches and giving us smarter targets. Enemies will stand there and take as many bullets as you can throw at them. Missions with your partner, Shane, don't fare any better. He'll run headlong into a hail of gunfire, getting himself killed and thereby failing the mission for you. And the fighting mechanics flat-out suck.