The arrival of a crate of pineapples and an odd pet at the Hollister home in Shoreham is the first of many unusual events in this new and exciting adventure.

The sender of the pineapples—Mrs. Villamil—is a girlhood friend of Mrs. Hollister and now lives in Puerto Rico with her Spanish husband, their son Carlos, and daughter Maya.

The Villamils invite the Hollisters to their home at Lizard Cove for the children's winter vacation. The trip turns out to be more than just a sight-seeing tour when Carlos shows the Hollister youngsters an old tower on the Villamil property where they discover a stone that proves to be one of the clues to a pirate's treasure.

Along with the search for the location of the treasure, and the sinister threat of two strange men who try to stop the Hollisters, there is the fun of learning new customs and a different language. Last, but not least, suspense runs high when the girls enter the contest for the choice of Pineapple Queen of Puerto Rico.