Polly, Lisa and Lydia make a brave return to the troubled underwater city of Fortuna, this time taking their friend, Matt, with them.

The inhabitants of the Southern Quarter are now suffering unbearable hardship from the oppressive Polizia.

Together with their Fortunan friends, the girls and Matt come up with a daring plan to overthrow the tyrannical Imperial Governor and his supporters.

But while Polly and Lisa have to use all their wits to try to avoid capture, Matt and Lydia enter a terrifying place from which no one has ever escaped...