A girl drawn to the aquarium where her father works meets two boys raised by dugongs, as strange calls from the sea connect them to a worldwide marine mystery.
Ruka’s encounters with Umi and Sora do not fit neatly into what the adults studying the ocean understand. The children experience a connection that the researchers can only approach from outside. The mystery makes the sea a living system with its own scale, rather than simply the backdrop to a summer friendship.
Ruka’s summer starts with a clash with a teammate and distance from her mother. Feeling that neither school nor home has a place for her, she goes to her father’s aquarium. Her meeting with Umi and Sora therefore offers a possible connection at a moment when her ordinary relationships have become difficult.