At the hut, Dr. Moore meets Joe, Captain Daniel’s friend. Joe explains that he went exploring for Indigenous American artifacts, fell from a rock, broke his arm, and had amnesia. An old Indigenous American man found and cared for him. Joe whispers his real name to Dr. Moore, who is shocked but agrees to keep his identity secret. Joe doesn’t want to return home to his uncle until he is fully recovered. Dr. Moore assures Mr. Alden that Joe is a capable and dependable handyman.

Captain Daniel takes the group back to the mainland. After the Moores leave, the children shop for dishes and supplies. At their house, the housekeeper, Mrs. McGregor, shows them new play clothes and shoes that Mr. Alden bought. They change and pack their belongings, including Benny’s bear, and say goodbye. At the dock, Jessie remembers to buy bread and milk.

Back on the island, Captain Daniel helps them carry boxes to the barn. The children set up their barrel-and-board table, unpack dishes, and eat their first supper: bread in bowls with milk poured over it. They wash dishes at the spring, which Henry notices overflows into a stream. In the darkness, Henry spots what appears to be a vegetable garden. That night, Jessie retrieves Benny’s bear when he asks for it. The next morning, everyone wakes in their new home.

At six o’clock in the morning, Henry quietly lets Watch out and walks to the spring, confirming that there’s a vegetable garden. Joe walks toward him and introduces himself, and he explains that Mr. Alden had Captain Daniel plant it for them in advance. He tells Henry that the peas are ready to eat, though nothing else is ready yet. When the other children join them, Jessie plans dinner around the fresh peas.

After breakfast, Jessie proposes a summer schedule including daily swimming, cooking, and either making things or exploring. Joe tells them about the grocery ordering system: They can leave lists in a box on the dock, and Captain Daniel will fill the orders. Jessie and Henry write a list for butter, bread, sugar, dog food, and four bottles of milk each day. Benny goes with Joe to see the dock box while the others pick peas.

Henry builds a dish cupboard with cloth doors while Jessie and Violet shell peas. Benny returns, reporting on the grocery box, and Henry tells him that he may take the orders to the dock and collect the groceries each day. Jessie cooks the peas with butter and salt for lunch. Jessie remarks that everything tastes better when they have worked for it. After organizing the new cupboard, they set out at one o’clock to explore the island.

On the beach, Benny sees water shooting from the sand. Henry recognizes the sign of clams and begins digging with a stick. The children get spoons and a dishpan from the barn. Watch joins the digging, helping Benny fill the pan. They cover the clams with wet seaweed to keep them fresh for the next day’s dinner.

Continuing down the beach, they discover a raft in the water that their grandfather has set up as a safe swimming area. At three o’clock, they change into swimsuits. Benny finds the water too cold and refuses to go in. Henry and Watch swim to the raft, followed by Jessie, while Violet stays with Benny.

Joe arrives in his swimsuit and sits with Benny and Violet. He points out different types of seaweed, naming rockweed and showing them a red specimen. While chasing a piece of red seaweed in the waves, Benny wades in and gets completely wet without noticing the cold. Joe explains how to press and dry seaweed on paper to make a collection, and Violet decides to start one. The children notice how cleverly Joe gets Benny fully into the water without realizing it. That evening, Henry puzzles over Joe’s extensive seaweed knowledge for a handyman.

The next morning, the children plan to continue exploring before cooking their clams for lunch. After Benny returns from the dock with groceries, they set out with a large kettle to collect discoveries. On the beach, they gather interesting shells and flowers.

Henry suddenly has an idea: They should create a museum in the barn. He explains the concept to Benny and the others, who are enthusiastic. They brainstorm exhibits including birds, flowers, seaweed, shells, and butterflies. Jessie suggests using the barn’s upstairs, and Violet begins a list of categories. As they explore, Violet wonders how they will identify their specimens without knowing the names.

Returning to the barn, they see Joe getting into the motorboat at the dock. They run to the dock and ask if he can get books from the library. Joe agrees, and Jessie explains that they need picture books to identify their finds. At the library, Joe recites the names of many books from memory, causing the librarian and a small boy to look at him in surprise.

Joe returns with his arms full of books. Jessie invites him to stay for their clam dinner, and though he hesitates, Jessie takes his response as a yes. Joe gets a block of wood from the woodpile near the barn to use as a chair and teaches Benny how to eat steamed clams. After the meal, Joe helps with the dishes. With the chores done, Jessie asks Joe to show them the books.