A documentary that plays like a thriller isn’t all that rare, but “The Cove” is a good example, generating real suspense and emotion over an important social issue. Produced by the Oceanic Preservation Society, one of whose founders, Louie Psihoyos, appears in and directed the film, it’s a investigation of an annual massacre of dolphins off the coastal town of Taiji, Japan—an event that’s been hidden from public view by locals and the government authorities for fear of the outcry it would cause, not just over the brutality of the slaughter itself but over the fact that dolphin meat tainted with toxic mercury is then distributed to consumers, often fraudulently, and even included in school lunch programs. The picture also places the situation in a broader perspective by connecting it to Japan’s efforts to lift bans on whaling and, most importantly, to the ineffectuality of the only organization delegated to make policy on such matters, the International Whaling Commission, in addressing the issue of hunting smaller cetaceans like dolphins and porpoises, as well as to the popularity of dolphin shows at “Sea World”-type parks, which has created a thriving, and lucrative, market for live animals.

This makes “The Cove” sound like a dry, good-for-you eco-message, but it’s far from that, because it has a dramatic protagonist in Ric O’Barry, the man who trained the dolphins used in the original sixties “Flipper” TV series but, over the years of living with them, realized that they were self-aware creatures that should not be kept in captivity for human amusement, especially since the noisy environment of a park necessarily places great strain on the animals’ acute sonar sense. For more than thirty years, therefore, O’Barry has taken direct action to free captive dolphins and end hunting of them, in the process becoming a pariah to the IWC and authorities who have frequently arrested him for his assault on “private property.”

It’s O’Barry who not only is the leading voice in “The Cove,” offering (along with scientists and others) testimony about dolphin intelligence and interaction with man, but who’s the real spirit behind the effort to uncover what’s happening at Taiji. After investigating the annual rite himself but kept from the cove where the actual massacre occurs by local authorities and threatening fisherman, he enlists Psihoyos and the OPS in what becomes a kind of Special Ops mission to secretly position cameras and listening devices in the sheltered area to record the slaughter and make the world—including most Japanese, who, man-on-the-street interviews show, are ignorant of the practice—aware of it.

The film is certainly an earnest brief on behalf of the dolphins and a celebration of the crew—environmental activists, surfers, even technicians at Industrial Light and Magic—who participate in the complicated, potentially dangerous effort to document what’s going on at Taiji. (Anybody caught can be jailed with charge for twenty-eight days under Japanese law and tortured into a confession.) It’s also a searing indictment of Japanese government policy, which not only silently condones the slaughter but apparently conspires in the distribution of tainted meat and perverts the work of the IWC by eliminating smaller cetaceans from the organization’s prohibitions and attempting to lift whaling bans with the support of small nations it bribes for their votes. And it places the pro-dolphin activists in a broader historical and ecological context by including archival footage of previous outbreaks of illness caused by mercury poisoning and the sixties campaign that led to the present ban on whaling, as well as interviews with scientists about the continuing deterioration of the undersea realm as a result of human activity.

But the centerpiece of “The Cove” is the mission at Taiji, and although the gritty night-time footage of the crew setting out the cameras and recorders doesn’t generate the sort of wrenching tension the best fictional heist movies can (the images are often indistinct, the topography unclear and the threat posed by security forces never viscerally felt), that’s to be expected when what one sees is dictated by reality rather than a screenwriter’s imagination and a director’s ingenuity.

And the murkiness of that sequence is, in any event, swept aside by the clarity of the horrifying footage those hidden cameras capture—of a cruel slaughter that literally turns the waters red with carnage. It can’t help but stir a viewer’s emotions and aid in what is clearly the picture’s final purpose—enlisting as many people as possible in a campaign to save the dolphins. (Information about how to join the movement is provided at the close.)

“The Cove” benefits enormously from Brook Aitken’s camerawork, some of it done under the most difficult conditions, Geoffrey Richman’s taut editing, and J. Ralph’s supportive score, as well as Psihoyos’ direction and the work of a large team that makes the sounds of the hunted dolphins as painful as the images of them being harpooned and hauled aboard the boats, dead and dying—images that make the sugar-coated descriptions of officials all the more odious.

“The Cove” is an unabashedly activist, one-sided piece of cinema, but a very effective one that probably won’t find wide distribution, though it should.