When a devastating flu-like virus wipes out most of the world's population, a handful of survivors are left struggling to stay alive as society falls apart around them. Abby Grant, a housewife and mother, is among those who remain, and sets out to find her 11-year-old son. Along the way she meets a variety of people spared by fate, and they must stick together if they are to survive this dangerous new world.

Abby and her new friends discover that supplies will not be shared equally in the new world, as they come face to face with a gang of armed thugs who have laid claim to the supermarkets in the area, leaving them short of food and water. Greg, meanwhile, meets a young woman with a tempting proposition for him which would leave him with a comfortable new life, but would also mean turning his back on the others.

Abby stumbles across a community that could hold the key to everyone's future. The group is led by Samantha Willis - the last surviving member of the Government. She has set up a community that still boasts light, hot water, food and power, all drawn from sustainable sources. Abby and Samantha hit it off immediately and Abby is full of hope that Samantha might lead the remaining population to build a new and better society. Samantha sees that Abby could become a trusted ally, but Abby's illusions are shattered when she sees the lengths Samantha will go to in order to maintain control of her fledgling community.

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.

Suspicions are aroused when visionary spiritual leader John turns up mysteriously at the survivors' door with his ragged group of passionate disciples. Is he as innocent as he seems, or does he have more sinister intentions? When Abby allows a pregnant woman from John's group to stay with them until the baby is born, Anya fears that she will be forced to reveal that she is a doctor, and she becomes concerned for the group's safety after some unusual remarks from John.

Abby and her friends find themselves on the run from Samantha and Dexter, who have 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.