With the help of Richard O'Barry, a former dolphin trainer who has since recanted and become the mammal's strongest ally, filmmaker Louie Pshihoyos sets out to expose the illicit slaughter of large numbers of dolphins at Taijia, a rural Japanese cove.
The pair have to contend with bureaucratic obduracy, police surveillance and attacks by the fishermen. Forced to film undercover and underwater, they use Industrial Light and Magic's latest technology to capture the heartrending massacres.
Psihoyos links events at Taija to wider concerns: the lucrative global aquarium industry, which needs trained dolphins; the impotent regulatory checks in place; and a whaling industry that is flexing its muscles again.
And there is an ironic coda to the slaughter; the dolphin meat is relabelled as whale meat and is particularly popular in children's lunchboxes. The meat is high in poisonous mercury toxins.