Darwin Island sits at the far northern edge of the Galapagos archipelago, a scrap of volcanic rock barely a square kilometer wide that somehow draws a recurring parade of whale sharks, the largest fish alive. Naturalist and photographer Jonathan Green leads a team trying to find out why, and what they notice first is that nearly every shark passing through is a pregnant female. To track where they go afterward, researchers attempt to fix satellite tags to animals that can weigh many tons, a job complicated by steel spear tips that bounce off tough hide, unpredictable currents, and tail strikes strong enough to break bone. The footage follows those attempts underwater at close range, alongside the wider effort to build what the team frames as the largest study yet attempted of whale shark migration. Green compares the situation to finding dinosaurs still walking the earth unrecognized: an animal this size, moving these distances, and almost nothing on record about its mating or birthing habits. The film also notes the pressures now closing in on the population, from fishing to habitat disruption.