Subtitled 'A Holiday Adventure of John, Mary and Anne Arundel' the film begins with two girls and a boy playing in the sea. The boy finds a message in a bottle that they discover is a treasure map thrown overboard by a sailor in 1780. They follow the clues on the map and find themselves outside a house known as the Mermaid. No one answers their knocks so they force the door open and go inside, unaware that they are being watched and then followed by an old woman. The map leads them down into the cellar where they find a secret door leading to a passage. Having entered the passage the old woman locks the door behind them. The children continue, not knowing that their escape route has been barred. The passage leads to a cove and then a ladder, which the boy climbs. At the top he finds himself in a small room containing a chest full of jewellery. He lowers it down to the girls and they make their way back along the passage to the locked door and panic ensues. Meanwhile, at the beach the tide is coming in and the children's mother starts to worry about them. She searches for them by car and arrives at the Mermaid house. On answering the door the old woman denies having seen the children. The mother hides out of sight until the woman leaves the house and breaks in. At the same time the children fear for their lives as the sea water begins to rise. The mother finds the door in the cellar and, in the nick of time, opens it, saving the children.
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