The answer is that you canít. Since his/her victims were random, itís tough to follow any one of them. Since the detective assigned to the case got nowhere, his story isnít very interesting. Unfortunately, we follow the story through the eyes of the detectiveís son. Thatís where Rory Culkin comes in. Rory does a great job, but itís a point of view that just doesnít work for a serial killer movie. I appreciate the attempt to give a new perspective on a serial killer, but this one doesnít work. In fact, the story of the Zodiac doesnít work at all. The killer had no pattern, no rhyme or reason for his/her victims and has yet to be caught. Thereís just not enough material here to make a film.
That fact is evidenced throughout the film. The filmmakers try to focus on the detectiveís family and the journalist covering the case, but itís not enough. Itís creepy, sure, but I need more in a psychological thriller than the occasional willies. Also, the director didnít do enough to scare or creep out the audience. There werenít any intense moments except for the cheap thrill early on in the kitchen. Other than that, we have a linear tale of a deranged killer. I donít find serial killers interesting, but I take comfort in the fact that they usually die at the end of the movie. Unfortunately, this one hasnít even been caught. If you know that going in (itís plastered all over the cover) the movie wonít provide you with much suspense.
Behind the Zodiac(11:03): You get to see the guy (who knows if it really was a guy?) that played the killer. He offers nothing. This is mainly a collection of interviews with the cast and crew. Itís decent enough, but not great.
Zodiac Letters, Decoding the Zodiac, Chronology of the Zodiac Killings: These are collections of text that are pretty self-explanatory. If you have the time, and youíre curious, they might be interesting for you.
There are also some Previews.