There are horrifying moments aplenty in Gabriela Cowperthwaite's advocacy documentary Blackfish, but the one that hit hardest for me was when one of the movie's many talking heads (apologies for the paucity of my note-taking, there are a lot of people and very few of them get identified more than once) recalls the story of one of SeaWorld's female orcas and her calf. The juvenile orca was taken to another SeaWorld facility, severing the family relationship at the heart and soul of orca social behavior, and almost immediately after her child was airlifted out, the mother began making a vocalisation that none of the trainers recognised. A cetacean researcher was brought in to consult, and revealed that this was the sound of an orca's long-range communication. You don't need to be a romantic or have a bad habit of anthropomorphising animals to find the thought of a female cetacean calling across an unknown, insurmountable distance to find her child to be the most tragic damn thing ever.

To her immeasurable credit, Blackfish director Gabriel Cowperthwaite doesn't chase this sort of thing any further than happens in the natural course of the film. Unambiguous message movie though it is, Blackfish doesn't go for cheap emotional highs to get the audience where it wants us to be: it is a film that would much rather convince us through testimony and facts than by hauling out weeping orphans to make us feel just simply fucking awful about life. Again, some of that happens along the way naturally; the subject of the film is an immensely depressing one, and many depressing events are recounted. But they are recounted as evidence in an argument, not as misery porn, and it is to the film's absolute and unmistakable benefit that this is the case (it is, disappointing, however, that nobody thought to tell composer Jeff Beal before he turned in a weak assortment of dribbly, crudely manipulative light jazz cues, resembling the ambient music in a chiropractor's office more than a movie score).

The subject of the film, of course, is orcas; and not just orcas as a concept, but orcas as the feature attraction at theme parks on the SeaWorld model, and especially the ways that they murder people. The film was begun in the wake of an incident in February, 2010, in which Dawn Brancheau, a 40-year-old orca trainer at SeaWorld Orlando, was pulled into the water and killed by Tilikum, the largest captive orca in the world. Cowperthwaite, having a brain in her skull, didn't buy SeaWorld's official story that Brancheau was dragged in because the animal; mistook her ponytail for a toy, and this kicked off a fine and massively persuasive piece of journalism.

Brancheau's death is both the inciting incident of the film and the subject to which it steadily advances; without ever specifying exactly where things are headed, Cowperthwaite drops just enough hints in the early going to make it clear that her story is about the chain of events that made Tilikum not simply a killer, but a three-time killer: 19 years prior to Brancheau's death, almost to the day, he was one of three orcas to drown a trainer at Sealand of the Pacific in British Columbia, and in 1999, a dead tourist who'd illegally sneaked into SeaWorld at night was found mangled and dead, draped across Tilikum's back. What absolutely nobody in the the entire movie proposes is that this is just a matter of a killer whale living up to his name. The actual suggestion, forwarded by multiple people with different phrasing and degree of alarm, is that the horrible mismanagement of his multiple theme park homes, including the simple fact that large aquatic animals noted for their social behavior and thousand-mile ranges absolutely should not be kept in concrete pools, has literally driven Tilikum insane.

Making that argument is a two-pronged affair: first it takes arguing that orcas are intelligent enough with the kind of emotional lives where "psychotic" makes even the remotest sense as a diagnosis, and then one must show how the decades of captivity have broken Tilikum. Cowperthwaite manages both of these tasks very well indeed, with the caveat that the film itself is aware of (though it never comes right out and says it) that we can't ever really know what goes on inside a cetacean's head, for even if they are intelligent and sentient, their way of existing in the world is too removed from our own. Still, a cavalcade of biologists and ex-SeaWorld employees go a long way to prove that orcas are pretty special creatures, and as that idea is explored and deepened throughout the film it becomes increasingly easy to believe the vaguely silly notion of whale insanity.

It's almost pathetically easy for Blackfish to mount the other half of its argument: the issue isn't proving that SeaWorld and similar institutions do things that are not in the whales' best interest, but in finding anything that such facilities do that can be even plausibly defended. A large portion of the film's first half consists of nothing but the former trainers giving their biographies, and in something as simple as that revealing how poorly thought-out the entire edifice of aquatic zoos must be, if such well-meaning, barely-competent people could be thrust into positions that put them in immediate contact with large carnivores after what doesn't even sound like minimal training (the look of worn-out despair on the middle-aged ex-trainers' faces in contrast with their bubbly youth, as seen on old video recordings, is the most damning visual evidence in the entire film). And it doesn't take too many shots of Tilikum's limp dorsal fin or descriptions of how he and the other orcas were handled as a matter of course to realise what joy-starved lives these creatures lead; the really damning stuff, including the horrifying image of the orca, beached on a concrete shelf and sexually stimulated to harvest his sperm, is just gilding the lily at that point.

All told, Blackfish is one of the most impressive, rage-inducing, heartbreaking advocacy docs I've ever seen. The only problem is that, at the end of the day, it's still an advocacy doc, and they're really just not very interesting things. Mounting morally outstanding arguments well is useful and all, but parading an army of talking heads interrupted with only the occasional graph is hardly the stuff of rich cinema, and the handful of visual coups - skillful interpolation of old SeaWorld ads, one sequence recorded from multiple tourists' cameras in which a trainer is almost drowned by Tilikum several times over several minutes - are really only enough to call attention to just how bland the rest of the film is. It's the nature of the beast, of course, and an artistically bold advocacy film would at that point probably cease to be very good at advocating, but Blackfish isn't the kind of movie you need or want to see a second time, once the first time has got you good and mad and ready to change the world.