The Cay takes place in 1942, starting on the Caribbean island of Curaçao during World War II. Eleven-year-old Phillip Enright, initially excited about the war, witnesses its harsh reality when German U-boats attack the S.S. Hato, the ship he's traveling on. The ship is torpedoed, and Phillip suffers a blow to his head. He finds himself stranded on a liferaft with Timothy, an old Black man, and a cat called Stew Cat. Upon waking the next morning, Phillip finds that he has turned blind.
His blindness makes him entirely dependent on Timothy, despite harboring prejudiced views instilled in him by his mother. However, Timothy proves essential to their survival by finding food and water and teaching Phillip essential skills like fishing and navigating. As they face the challenges of the open sea, a bond of trust and respect begins to form, slowly dismantling Phillip's prejudices.
Eventually, they land on a small, deserted island called "The Cay." Timothy builds a makeshift shelter there. He teaches Phillip to identify edible plants, face dangers like sharks and storms, make shelters, fish, weave vine mats, and climb coconut trees. Through these experiences, Phillip not only adapts to his blindness but also develops self-reliance and resourcefulness.
Their peaceful existence is shattered when a hurricane ravages the Cay. Timothy saves Phillip’s life by using his own body as a shield. Timothy ultimately dies due to exposure. Devastated but determined to honor his friend, Phillip sets out with Stew Cat to find a way back home. He builds an emergency signal using black smoke, which causes navy vessels to come to their rescue. Following his rescue, Phillip undergoes medical treatment and recovers his sight. He carries the profound impact of his experiences and the memory of Timothy's friendship, vowing to explore the world and one day return to the island that forever changed him.