Sunday and Sis spend each summer near the ocean in a little beach house. When Sis is over her head in the water Sunday rescues her. Again when one of her sandals is stuck in a root on the sand dunes it is Sunday to the rescue. They visit the lighthouse and Sis finds an ancient telescope. She visits the nearby Maritime Museum and finds another one just like it with the date it was made on the outside of it. They row too far out into the bay and find the tide pulling them out to sea. They are rescued by the lighthouse keeper. Sis's father tells her he will put a small engine on her little rowboat so she can explore up the river the next summer. That will be a story called Sunday and Sis Up the River.