Britain is in the grip of a deadly flu virus, which is now spreading throughout the world. When people start dying in their millions, a handful of survivors are left struggling to stay alive as society crumbles around them.

Abby Grant is a housewife and mother who begins the desperate search for her son. On her journey she meets a variety of people whom fate has spared.

The new world could be a dangerous place, and their only hope is to stick together.

Abby stumbles across a community that could hold the key to everyone's future. The last surviving member of the government, Samantha Willis, has set up a community that still boasts light, hot water, food, and power. Abby and Samantha hit it off immediately, but Abby's illusions are shattered when she sees the lengths Samantha will go to in order to maintain control of her fledgling community.

Meanwhile, Greg and Tom are out searching for supplies when they encounter a family who have successfully sheltered from the virus. When the daughter of the family reaches out to Tom and Greg, they are faced with a terrible dilemma, as any contact with them could kill the family.

Abby learns of a group of young boys living at a nearby mansion called Waterhouse. She leaves her new friends in order to find her son, Peter. When she arrives at Waterhouse she walks into the middle of a land dispute between an ex-army officer and an aggressive group of teenage boys. She must try and broker a truce in order to discover if Peter is at the house.

Tom, Sarah, Al and Najid are drawn to the comfort and security that Samantha's community provides. Things quickly go wrong for Al as he is ejected from the community. Tom sees the chance to prove himself to Samantha, but will his criminal past catch up with him?

Greg and Anya have an encounter with some unwelcome visitors that provides a vivid reminder of how vulnerable they are without the group.

In the final episode of the series Abby and her friends find themselves on the run from Samantha, who has formed a dangerous alliance that threatens everyone's freedom. Meanwhile, Sarah is confronted by a face from the past, which leads her to betray her new friends.

When Naj goes missing, the family face the danger of the city in order to find him. Naj is befriended by a feral boy named Darren. Darren lives with the sinister Craig and June, who look after a group of children who forage amidst the ruins of the city in return for the shelter and protection.

With his adopted family searching for him in the deserted city, time is rapidly running out, and Dexter is only a step behind them all...

A passing trader betrays the family's new location to Samantha, who arranges to have Tom captured and tried for the murder of a member of her community.