Book starts off immediately after the previous finished. Callie has just seen Gracie ( her little sister) and Matt ( her boyfriend) absorbed by the Change and she is left all alone in The Zone where the Change rules. Suddenly, her father appears from nowhere but she doesn't trust him entirely as he has experienced The Change but his mind seems to be his own, which shouldn't be possible. Her father takes Callie back to his old lab at the University and tells her that he had worked out an immunisation to The Change but administered it on himself too late to stop it effecting him. He also drops the bombshell ( which the readers suspected all along) that he immunised Callie in time. Therefore, she won't metamorphisize but she will feel the Change around her.
Then people who have experienced the Change start to chase her and her father appears to save her - but really can she trust him?
SPOILER ALERT ; She meets some new unchanged people living in an Ark underground ( hence the title). They are trying to preserve every unchanged species on the planet and the mad doctor in charge has decided to incinerate everything on the surface and start again where as Callie believes her blood will provide a better solution to The Change in being an antidote. The book ends with a cliffhanger.
NOTE : I HATE THE COVER!! The first book had a much better one. This is not as good as the first as there is a bit too much happening and this may confuse the kids reading it. ( )