Ollu is a barger; a trader living and working on her mother's matriarchal boat, The Ark.

When they lose all their trade goods in a storm and her Mum gets sick, the only way to save her mother's life and the life of her baby siblings is to make the most dangerous trade of her life. Ollu has to venture into forbidden waters, Bad Water, and she must go alone.

With her old allies under attack, she finds herself reluctantly welcoming two escaped slaves on board The Ark. Buzz is a genetically enhanced stranger from across the sea, while Ratter is a boy prophet, a spy from the old City. The Ark is forbidden to males but she has to accept their help. How many rules will she break to save her mother? Is she prepared to risk everything?

In a world reshaped by floods and the loss of technology, Ollu must make a perilous journey. She is pitted against gang leaders, slavers and violent machete-men. Only her courage, unexpected friendships and rediscovered technologies can save her mother's life - and her family's honour.