In the not-too-distant future, Scotland is ravaged by a deadly virus known as the Reaper, leading the government to quarantine the entire country by erecting a massive wall to contain the outbreak. The rest of the UK watches from a safe distance as those trapped inside fall victim to the virus or are left to fend for themselves. The isolation is so absolute that all communication with those inside ceases, and Scotland's fate becomes shrouded in mystery.

Years later, the Reaper virus resurfaces in London, causing panic and fear of a new epidemic. Desperate to find a cure, the authorities send elite soldier Eden Sinclair and a team of specialists back into the quarantined zone. They are tasked with finding a scientist who was working on a cure before the wall went up and is presumed to still be alive. As they embark on their dangerous mission, they discover that the survivors inside the zone have descended into a chaotic world of marauder gangs and medieval societies, where the strong prey on the weak and survival is the only law.