“Blackfish” opens Friday at Sundance Cinemas. PG-13, 1 hour 23 minutes, three stars out of four.

There are moments in “Blackfish” as suspenseful and scary as in any horror movie you’ll see this summer. Take, for example, the chilling sequence in which a killer whale takes a veteran SeaWorld trainer’s leg in his mouth and drags him down to the bottom of the tank. He looks like a beagle with a chew toy in his mouth — you can’t tell whether the whale is being malicious or being playful — and the whale surfaces just long enough for the trainer to catch his breath, his consciousness fading, before dragging him down below again.

That trainer survived, but others weren’t so lucky. Gabriela Cowperthwaite’s disturbing documentary isn’t meant to titillate us with such sequences, but to show audiences the moral and physical cost of keeping killer whales in captivity — a cost paid by both the whales and their trainers.

The movie centers around the 2010 death of another veteran trainer, Dawn Blancheau, who was dragged under water by a male whale, Tilikum, during a performance, mutilated and killed. (One trainer vividly remembers being told that day of her death, and then the chilling words, “He’s still got her.” SeaWorld insisted that the death was the result of an error on Blancheau’s part, first circulating the erroneous story that she fell in the water, then that Tilikum grabbed her by the ponytail.

Both stories are belied by the videotape (which “Blackfish” mercifully spares us), and the Department of Labor’s Occupational Safety and Health Administration successfully sued SeaWorld. Now, trainers have to be separated by a barrier when they work with orcas — SeaWorld is appealing.

But “Blackfish” digs back into Tilikum’s history, and finds that Blancheau was the third trainer Tilikum had killed over a 20-year period. Contrasted with cheery promotional videos of SeaWorld from the ’80s and ’90s, we’re presented disturbing testimony of how Tilikum and other killer whales have been treated in captivity — held in concrete pools, separated from their parents, sometimes brutalized by the more dominant female whales.

Much of this testimony comes from OSHA, but much of it also comes from former trainers, once cheery spokespeople for SeaWorld, now feeling disillusioned and betrayed by the company. The upshot of the film is that killer whales don’t attack trainers because of some predatory nature, but because they’ve been psychically warped by such treatment.

SeaWorld declined to be interviewed for “Blackfish,” a decision they may be regretting, now that the company has tried to launch an aggressive counteroffensive against the film. It is undeniably true that millions of people have walked out of a SeaWorld park over the last forty years with a greater appreciation for marine life. It is also undeniably true that those people have also spent a lot of money at SeaWorld. The question that “Blackfish” provokes, viscerally, is whether the education and entertainment for visitors, and profits for the company, are worth it if the animals are mistreated and unhappy. After watching “Blackfish,” the answer is crystal clear.