Most vampire movies try to find a new approach to the subject, with varying results. Some keep to the traditional view of the vampire as a supernatural monster and some incorporate more modern medical theory. Daybreakers falls in the second category. Sort of.
In a not too distant future a pandemic has hit the planet, turning most regular citizens into vampires. You still get infected through blood and biting, but the vampiric condition has become the norm rather than the exception. This means blood is rapidly becoming a commodity that is in short supply. Some humans are kept like cattle in large facilities and their blood is ”harvested”. It's all done reasonably humanely, the humans aren't awake for any of this.
Scientists are working on a blood substitute that is supposed to help with the supply and demand problems. The main blood supplier is the Bromley Marks company run by Charles Bromley (Sam Neill). Our main protagonist Edward Dalton (Ethan Hawke) is a haematologist hard at work on the task of finding a cure. He is also a reluctant vampire, having been turned by his brother Frankie (Michael Dorman). Frankie works as a human hunter, trying to round up any strays that may still be running around in the daytime.
There are some humans fighting the disease, trying to stay ahead of what is rapidly becoming the rule. These are led by Audrey (Claudia Karvan) and Elvis (Willem Dafoe). Elvis has managed to cure himself from vampirism through an extraordinary set of circumstances that involve a car crash, sunlight, and water.
The vampires are starving, which is why the clock is ticking for Edward to find a cure as well as for the whole vampire population in terms of survival. If they feed on other vampires, on themselves, or on animal blood for too long they turn into a more primal, bat-like creature with a much higher level of aggression. They lose any remnants of humanity through the starvation process and are therefore summarily executed by the government.
There are things about this movie that are really appealing, like the way the world has adapted to nighttime living and what kind of technological solutions have been worked out to allow the vampires to go about their business during the day. The heavy noir feeling you get from the grey, monochromatic life of false daylight and nighttime is contrasted by a richly suffused palette for the daytime scenes, which makes it easier to understand why Edward fights his vampire condition so hard.