Sena is you's classmate. She's the most beautiful girl in the class.
They're just ordinary classmates, neither particularly close nor distant, and have never had a deeply personal conversation.

During a school trip, the ship carrying the class capsizes. you instinctively shields Sena, who was nearby, and they fall into the sea together.
When you regains consciousness, they've washed ashore on an unfamiliar beach with Sena.
Exploring the area together, they find no other people. The only signs of civilization are some trash washed up on the beach, with no buildings in sight. It seems they've drifted to an uninhabited island.
While comforting the bewildered and dejected Sena, you begins a survival journey to make it back alive.