A new strain of the flu virus emerges, with a devastating effect on the population. Those who survive the epidemic must adapt to a new world in which government has collapsed and the old certainties have disappeared. A disparate group of survivors find themselves drawn together in adversity.

A small group of survivors find a house in which to shelter and determine to gather essential supplies, but a visit to a supermarket proves dangerous. Abby, determined to find her son, Peter, thinks she sees his teacher. Greg sets off in search of the main supermarket warehouse, but finds that other survivors have got there first.

Abby comes across a viable community led by Samantha Willis, the former junior health minister and the last surviving member of the government. Greg and Tom, searching for supplies, discover a family that has survived by remaining in isolation during the outbreak.

Abby sets off for Waterhouse, a mansion where, it is rumoured, a group of boys has taken refuge, but finds herself caught up in a territorial dispute. Tom, Sarah and Najid soone settle into Samantha's community, but Al is ejected. Greg and Anya, alone at the survivors' house, have some unwelcome visitors.

The harmony of the survivors is disrupted when they are joined by a spiritual leader and his followers. Al attempts to seduce one of the newcomers. Anya worries that she will be forced to reveal her secret.

The survivors go on the run in an attempt to escape Samantha and Dexter. Naj, separated from the group, joins a gang of children foraging in the abandoned city. Sarah's loyalties to the group are tested.