In Vieques, Gabe is chasing the science and stories behind one of the world's brightest bioluminescent bays. Iker, a fisherman's grandson who knows the mangroves and tides better than any visitor, warns the triplets that glowing water is beautiful—but never simple. Then a drumbeat crosses the dark bay where no drummer should be.

The sound leads toward a forgotten warning route and a mystery buried beneath the bay's blue-green light. Tala must map conditions that keep changing, Naya must listen to the people behind the legend, and Rafi must decide whether the water is echoing the past or answering the present. Their search for their missing mother is about to take another unexpected turn.