While the rest of her family sleeps, a young girl takes her little brother--whom she knows will be frightened by the windstorm raging around their seaside house--on an imaginary voyage with his favorite animal, the humpback whale. The bed lifts and the waters welcome them as they embark on an undersea odyssey to swim and breach with the gentle creature who rocks them gently to sleep in a rumbling, shimmering hammock made of waves.