An unidentified virus wipes out most of the Earth's population, liquefying their bodies. The virus moves as if it has an agenda-- it adapts as it moves quickly across the planet. Leon, his mother, and younger sister Grace, who just moved to London from New York, must run for their lives.

Eighteen months after the plague struck, Leon and Freya hope two hard winters killed it off, but when they set out for a rescue ship they find the virus has been busy.