The Outdoor Girls on Pine Island, or, A Cave and What It Contained is a novel by Laura Lee Hope, published in 1921. It is part of the "Outdoor Girls" series of books. The story follows the adventures of a group of friends: Mollie Billette, Grace Ford, Betty Nelson, and Amy Stonington. The girls are excited to spend their summer vacation on Pine Island, where they plan to swim, hike, and explore the island's many caves. However, their plans are disrupted when they discover a man living in one of the caves. The man, who calls himself "The Hermit," tells the girls that he is hiding from a dangerous criminal who is after him. The girls decide to help the hermit by keeping his presence on the island a secret and bringing him food and supplies. As they try to solve the mystery of the hermit's identity and keep him safe, the girls also have other adventures on the island, such as exploring the caves and dealing with a group of troublesome boys who are camping nearby. In the end, the girls are able to unravel the mystery of the hermit's past and help him start a new life. They also have a fun and memorable summer vacation on Pine Island, filled with adventure, friendship, and new experiences.