A signal sent out over the radio, TV, phone, etc. turns those exposed homicidal. They aren't zombies, they still have their personality, they're just obsessed with killing and see it as a completely rational act.
Told in three parts and following a young woman, her abusive husband, and her love interest, this is a low-budget, tonally slippery tale. Violence is gruesome, but worse is hinted at offscreen in all three acts; and the second adds dark, almost absurd humour into the mix. Not sure it needed to be 1hr43min long, but the fake-outs and strangeness kept me watching.