Remember that all the fun you had playing that zombie video game where you played as two zombie-killing sisters, one clad in a catholic school girl outfit and the other dressed in a bikini, cowboy hat and feather boa? Of course you don't, and that's because the Onechanbara game series was only a hit in Japan, where it spawned several sequels, the last of which was unceremoniously dumped into the U.S. market to some of the poorest reviews a game has ever deserved.
The movie version was released in Japan in 2008, and has already received a sequel, while Tokyo Shock picked up the title for release in August 2009. While the movie adaptation doesn't require you to know the game, per se, the movie would sure make more sense if you did. For instance, in the movie, Aya (Eri Otoguro), the cowboy hat, feather boa-wearing sister is the big bad-ass zombie killer to start the movie, while sister Saki (Chise Nakamura) is not her ally. In fact, Saki is the very person Aya is seeking so she can take her revenge on Saki for her betrayal of their father. Instead of Saki, Aya is joined by game character Reiko (Manami Hashimoto), who helps bail out Aya with her always-loaded shotgun, and a fat idiot with dyed-blonde hair who is there simply to add comic relief. At first I objected to this comedic ploy, until I remembered that a fat person plays the comic relief in 90% of American movies.
The real detriment in not knowing the game is watching the end sword fight, which shows Aya and Saki using some of kind of force power emanating from their swords. This is an element from the game, of course, but seems like a random detail that only a Japanese movie would include. And frankly, despite it's game origins, it is.

With the ending spoiled, what's up with the rest of the movie? Brief nudity and poorly played zombies that are really just there for Aya to destroy with her katana, which often consists of her swooping her sword in a circle and watching the zombies explode in cheap-looking CGI.
Director and co-writer Yôhei Fukuda certainly tries to put together an entertaining 80 minutes — boobs and a scantily-clad heroine isn't exactly heading in the wrong direction — but the movie is more concerned with looking like the video game, which, as I pointed out, means almost nothing to anyone outside of Japan.
It's a shame really. I'm not sure I want to live in a world where movies about girls in bikinis who kill zombies aren't a sure-fire success, but, sadly, Onechanbara misses the mark.