Downfall is the story of how the USG Ishimura found itself in the predicament you have the pleasure of exploring throughout the game. After a routine mining operation, the crew discovers an alien artifact and brings it aboard. The artifact means different things for the crew and these feelings play a huge role in the overall plot. For members of a religious group, the Unitologists, the artifact is a holy relic and proof of the existence of God. For others, it is a curse and someone hid it on the remote planet for a reason. Their suspicions are confirmed when members of the crew begin to exhibit signs of maniacal behavior.
For anyone worried about Downfall giving away too much of the game's plot, there is little need to worry. The movie ends at the exact moment the game begins and within the first few chapters, any of the information given in the movie is revealed through video and audio logs. Besides, it doesn't take much to guess how it ends anyway.
At its core, Downfall is a horror movie, complete with all the gore and logical leaps you would expect from the genre. Actually, in some cases, the gore goes beyond what is usually seen in horror movies (with the exception of the whole "Torture Porn" movement). Some of the scenes are intense and a bit unnerving. If you played the game, you'll already know what to expect, but if you haven't, know this isn't a kids' cartoon.
Downfall isn't light on extras, which is surprising. There's the requisite promotional trailer for Dead Space, as well as one for the movie. The photo gallery is interesting for would-be artists or anyone interested in animation. Production art for the game is also viewable, so anyone interested in getting into the industry as an artist may also want to take a look. In that respect, the entire movie is worth watching just to see how well the game and movie worlds mesh. Artists, or anyone interested in production, will also want to check out the deleted scene, which is presented in storyboard form. Finally there's the isolated soundtrack, where you can hear the movie's soundtrack along with a few production stills.
I was able to watch the movie on both DVD and Blu-ray, and given the choice, I'd go with the Blu-ray copy only if you're really into picture quality and animation. Compared to the DVD, the colors in the Blu-ray version have a little more "pop" to them and some of the gradient problems I noticed in the DVD version (mostly around shadows) aren't around.
Again, I'll admit that I wasn't a believer at first, but Dead Space: Downfall is a great addition to the game's mythology and worth watching even if you already completed the game. Of course, if you haven't played the game, the movie will more than likely make you want to dive into it, which is as good an advertisement as any.