There’s a harrowing sequence midway through “Blackfish,” director Gabriela Cowperthwaite’s controversial new documentary about the practice of keeping killer whales in captivity, that underscores the film’s theme far more effectively than much of its “talking head” testimony.

In video footage of a SeaWorld aquatic performance, a trainer is swimming with an orca. Suddenly, the trainer’s foot is clamped in the huge mammal’s mouth and he’s carried down deep into the whale enclosure. The whale rises to the surface, the man’s foot still in its mouth. And then it takes him down again. It lets him up, and releases the trainer’s foot.

And then it switches to the trainer’s other foot and dives again, deeper this time.

Throughout the ordeal, the trainer, who shows superhuman composure under the circumstances, speaks to the whale and strokes its side until, just as suddenly, the animal releases him long enough for the trainer to swim to safety in front of the horrified crowd, his feet a bloody mess.

The whale swims after the man and slides up on the performing platform, just as it has done a hundred times before.

A voiceover from another former SeaWorld trainer explains that the trainer involved was one of the most experienced in the world, the whale in question never known for aggressive behavior. The two had worked together in seeming harmony for years.

It’s just something that happened.

“Blackfish,” showing this weekend at Space Gallery, has been the subject of the sort of all-out media attack only a massive corporation like SeaWorld can muster, and it’s easy to see why.

The film interviews orca experts, scientists, relatives of trainers attacked inside SeaWorld and other parks — and an eye-opening number of former SeaWorld trainers who cast serious doubt on the company’s capture and containment practices, training protocols and, most damning, the corporation’s PR response to any incident.

A disclaimer at the end of the film reveals that SeaWorld declined to be interviewed for the film. It was probably a smart move, as “Blackfish” clearly has as its agenda the abolition of orca captivity and especially performance.

And while the film is hardly objective about its goals (and uninterested in presenting much in the way of opposing viewpoints), it is most effective in countering the industry’s carefully crafted company line that the practice is humane and safe.

Especially when it comes to a whale named Tilikum.

Captured as a baby in the North Atlantic, the soon-enormous whale started out as an attraction at a low-rent Canadian park where, as one former trainer speculates, the truly cramped conditions (complete with abuse from other penned-in whales) may have damaged him. One tearful employee speculates that he was driven to “psychosis.” One day, Tilikum unexpectedly killed a young trainer.

The park closed down, and Tilikum was sold to SeaWorld where, the film makes a convincing case, the circumstances surrounding his arrival were hushed up. One morning, trainers found the dead, partially devoured body of a young man in Tilikum’s tank.

The park ruled it a drowning — some nut who wanted to swim with the whales. Then in 2010, Tilikum became infamous when he dragged down and eventually devoured experienced trainer Dawn Brancheau during a show, this time in front of another horrified crowd of tourists.

Throughout, Tilikum is presented as the poster-whale for those who believe that orcas — presented as sensitive, intelligent, family-oriented creatures with a sophisticated social structure — have no place being hauled in to entertain tourists with silly tricks. And that Tilikum’s checkered history was the inevitable consequence of doing just that.

Unfortunately, while the film is never less than compelling, emotionally wrenching and, in the intermittent attack scenes, undeniably upsetting, it’s also very thin. While SeaWorld chose not to take part, Cowperthwaite doesn’t bring in anyone to seriously debate a single point made by her own experts. “Blackfish” is a polemic against the practice of keeping these animals in captivity, and while there’s ample emotional appeal and anecdote here, a more balanced presentation would strengthen the film’s case.

Going back to the first incident I mentioned, I’m reminded of comedian Chris Rock’s response to the time that a trained big cat attacked one half of Siegfried and Roy: “That tiger didn’t go crazy — that tiger went tiger.”

“Blackfish” might not make an airtight case against training wild animals for our enjoyment, but it very convincingly argues that those animals remain what they are, no matter how SeaWorld or any other corporation invested in presenting them as cuddly attractions tries to say otherwise.

We might constrain nature for our enjoyment — but sometimes, the show’s going to have blood in the water.

“Blackfish” screens at 7:30 p.m. Friday and Sunday at Space Gallery, 538 Congress St., Portland. Tickets are $8; $6 for students with ID. Go to space538.org for more information.

Dennis Perkins is a Portland freelance writer.