The world's tropical waters are inhabited by a fish that dwarfs all others. Yet it is fuelled by the tiniest of creatures. The whale shark spends its life pursuing rich upwellings of microscopic plankton. Off the east coast of Africa, the warm currents of the Indian Ocean carry with them a seemingly infinite supply of food. Each year, with the onset of spring, the gentle giants leave the equatorial waters of the Seychelles and follow these currents southwards, on a remarkable journey, unique in nature, thousands of miles along the ocean's highways. We follow one shark all the way to the southernmost tip of the continent. Scientists do not yet fully understand this phenomenon but we seek to unravel the mystery of the whale shark's incredible journey. Spectacular up-close footage, combined with the latest scientific research, sheds light on one of the ocean's great creatures.