Four children on a holiday at the seashore find out that the local circus has a mermaid captured and they decide to set it free. They organize a very brave rescue mission at midnight and manage to perform the escape. As a sign of appreciation, the mermaid takes the children to the hidden underwater kingdom for new adventures.
Edith Nesbit (1858-1924) was the author of world famous books for children - the tales of fantastical adventures, journeys back in time and travel to magical worlds.