Take a wild animal. Train it to perform tricks. Reward it with treats and applause. Ta-da. Meet a theme-park phenom in the form of the killer whales (orcas in the trade) on view in Gabriela Cowperthwaite’s gripping mindbender of a documentary. Just don’t even think of taking the kids to Blackfish. This eye-opening doc contains sights and sounds that are stuff of nightmares. The film brims over with stories and videos about the capture of orcas in the wild and how they are not suited to captivity. But the focus here is on Tilikum, a 12,000 pound stud orca involved in the death of Dawn Brancheau, who in 2010 was pulled underwater and drowned by Tilikum at SeaWorld’s park in Orlando, Florida. Trainer error? Exactly, say the suits at SeaWorld. But way back in 1991, Tillikum was also involved in a trainer’s drowning death at a park in Canada. Cowperthwaite adds more evidence to make her case, not just against Tilikim, but the whole business of what happens to mammals harnessed into entertainment slavery. Forget The Conjuring, Blackfish may be the scariest movie around.