About to turn sixteen, Nami revisits her only three birthdays that fell on February 29 - the leap day. Something happened on those days, of that she's sure, but the memories are vague and her mom is weirdly evasive. Determined to find out the truth, Nami turns to the notebook of her grandmother, a Japanese ama diver. By acting out the ama routine at the local YMCA, Nami hopes to rekindle the memories that stir in her unconscious. But the main ama practice - the breathing of the sea whistle - is a dangerous tool to a novice diver. And Nami has to wonder: are her visions real or simply due to lack of air?