Mario and his brother Alonso become committed to fishing in a coral-reef in the southern Philippines. With many other children of different origin they work for starvation wages on the ship of unscrupulous captain Murène. Together they drive shoals of fish into a big fish trap having to use the dangerous muro-ami technique in the coral-reefs. Fastened to a stone, the little divers jump into the water to reach the depth faster. Without any equipment they dive down 30 meters. The brutal captain treats the exhausted children like slaves