A prison van carrying serial killer Jack Frost to his execution is involved in an accident with a tanker carrying genetic research material. During Jacks attempt to escape, the experimental liquid in the tank sprays all over him, causes him to melt and some of his blood merges with the snow. So instead of dying, Jack becomes a snowman who is bent on revenge against the small town Sheriff who arrested him.
That about sums it up. The rest of the movie is Jack killing people and no one suspecting it is a snowman. Then when the Sheriff figures it out, no one believes him. There are deaths by icicle, Christmas tree decorations, and a beheading with a sled. So if you like wise cracking killers, and want to see a killer snowman, here you go. The comedy falls flat, but what the heck, how many times do you see a killer snowman.