At the end of Necropolis
the Five ran through the door out of the temple in Hong Kong as they made their escape from the Nightrise Corporation (which serves the Old Ones) and the deadly tornado that, as it turns out was the result of Scarlett's power. The doors take the Five to the destination they are thinking of as they pass through, but in their rush they ran through without thinking. Now they're separated again.
But there's a new problem.. the doors are locked and the world has changed! The Old Ones have meddled with time, and for the rest of the world 10 years have passed since Hong Kong. Nightrise and the Nexus are both waiting at every door for the Five to return.
Each of the Five must now fight to survive in a new world ruled by evil and overrun with poverty whilst trying to find the others. One of the Five will be easily turned against the others but will he see sense before it's too late? Sacrifices are necessary which will bring a tear to the eye of the more emotional/sensitive reader...