With the notable exception of The Godfather Part II, prequels have a habit of letting the franchise down. Think Psycho IV: The Beginning, or the dismal likes of Dumb And Dumberer: When Harry Met Lloyd. The very fact they need a subtitle to make them sound more interesting is a dead giveaway as to their lack of quality.
The major problem facing a prequel director is how to sustain our interest (and occasionally our disbelief) when we already have an inkling how the story is going to turn out. It's a dilemma that Andrew Lau and Alan Mak minimise in Infernal Affairs II by deliberately setting out to put the pieces in place for the original scenario - which they later decided was going to be the central segment of a trilogy (with III being filmed back-to-back with II).
Thus, by following Alfred Hitchcock's old maxim about telling the audience what is going to happen and then forcing them to wait for the pay-off, Lau and Mak have made the viewer (familiar with the follow-up) complicit in the various deceptions and showdowns, while also making certain actions and character traits from the first picture more comprehensible.
Considering how intricately I.A.II is bound into the original, it still makes for a satisfyingly accessible stand-alone thriller. Anthony Wong and Eric Tsang return as the brooding inspector and the small-timer on the make, while Shawn Yue and Edison Chen step into the shoes of Tony Leung Chiu-wai and Andy Lau, respectively as the undercover cop in the triads and the mob informer at police HQ.
But it's the new characters who give the action its impetus, most notably Francis Ng, excelling as the colonial Michael Corleone who reluctantly assumes control of the family firm, wipes out any potential rivals and then sets about diversifying his lines of business.
That said, the plight of the rookie moles still proves fascinating, as the focus on the fear of detection shifts slightly to the mechanics of survival and the professional traitor's notion of duty and honour. All we need now is a finale worthy of what has gone before.