The Earth passes through the tail of a comet, which dooms all human and animal life except for those who happened to be completely enclosed inside metal containers (for example, the three main protagonists spend the night in a metal shed, a steel-lined projection room, and an 18-wheeler's trailer). Unprotected people and animals quickly dehydrate and turn into piles of red calcium dust. Those only partially exposed first gradually degenerate into cannibalistic zombies for hours or days before succumbing.
cannibalistic zombies you say?
Dang, I need to make another run to the ammo store.
DAHdee would have got us Uzis !