The whole family is heading to the beach. Maruko is so excited she can hardly stand it, happily blowing up her float ring and bouncing around. But while playing in the sea, she drifts out farther and farther with the float still around her, giving everyone a terrible fright. Later, just as the sun is setting and they’re getting ready to leave, there sits the heavy watermelon they lugged all the way for a watermelon-splitting game…