One September evening in 1785, the merchant Jonah Hancock finds one of his captains waiting eagerly on his doorstep. He has sold Jonah's ship for what appears to be a mermaid.

As gossip spreads through the docks, coffee shops, parlours and brothels, everyone wants to see Mr Hancock's marvel. Its arrival spins him out of his ordinary existence and through the doors of high society, where he meets Angelica Neal, the most desirable woman he has ever laid eyes on... and a courtesan of great accomplishment. This meeting will steer both their lives onto a dangerous new course.

What will be the cost of their ambitions? And will they be able to escape the legendary destructive power a mermaid is said to possess?