Umi Matsuzaki is a sixteen-year-old student attending Isogo High School living in 'Coquelicot Manor', a boarding house overlooking the Port of Yokohama in Japan. Her mother, Ryoko, is a medical professor studying abroad in the United States. Umi runs the house and looks after her younger siblings, Sora and Riku, and her grandmother, Hana. College student Sachiko Hirokouji, and doctor-in-training Miki Hokuto, also live there. Each morning, Umi raises a set of signal flags with the message "I pray for safe voyages".