Eli Roth has called Fin the scariest movie he's ever made. He's right about that. The Hostel and The Green Inferno director dives into the documentary format for the first time, looking at the tragic loss of a conservatively-estimated 100,000,000 sharks each year due to the illegal practice of killing them for their fins. Like Roth's other works, it's pretty gory and occasionally difficult to watch. Nevertheless, the movie – which was co-produced by Leonardo DiCaprio – is essential. You come away with a much greater understanding of what it would mean for the ecosystem if they went extinct.

Early scenes describe the role of sharks in keeping the world's oceans healthy. We learn that they act as underwater police, helping to keep various other species out of places they should not be. Sections of the ocean where the shark population has been essentially eradicated are withered and dead. That's the short version of it. Fin goes into more detail as Roth interviews experts who put everything into perspective.

So why are so many sharks being killed? A big reason is shark fin soup, a food considered a delicacy in certain parts of the world. Fishing boats pull tens of thousands of sharks out of the water daily, cutting off their fins and often discarding the rest. Traders, frequently working in illegal ways, then sell those fins to restaurants, where diners pay to eat them. The film has plenty of graphic footage of sharks being callously maimed in this manner. If you aren't saddened and angered by the sight, you may not have a heart inside your chest.

Armed with such images, Roth goes on a mission to expose the people behind this nefarious business. At one point, he and Gary Stokes – the Director of Operations for OceansAsia – venture to Hong Kong, where they attempt to film inside public fin markets. The proprietors rush to lock their doors or block the cameras from seeing anything. Behavior doesn't get any more suspicious. Alarmingly, shark fins are sometimes dried out en masse along city streets. As Stokes says, people eat something that has been nestled up to cigarette butts, dog feces, and who knows what else. Aside from being terrible for the sharks, it's disgustingly unsanitary. The footage will blow your mind.

Most harrowing of all is an extended section in which Roth joins up with the Sea Shepherd Conservation Society, a group that monitors illegal fishing activity. They make their way onto one of the ships that's netting sharks. Conditions inside are deplorable, with shark carcasses strewn all over the place. The brutality of the practice becomes crystal clear when you see it. Not only are the fins desirable, but oil drawn from the livers of the sharks is sold to cosmetics companies for items you've probably rubbed onto your face. These same ships extract that oil in a stomach-churning way.

Fin is necessarily horrifying to watch. Thankfully, it ends on a positive note, with a recap of some significant strides made by people and organizations fighting to save the sharks. All hope is not lost. To feel that hope, you have to open your eyes to the rough stuff, though. Roth presents information in an engaging, urgent manner, and he brings great suspense to the section with Sea Shepherd. Clearly, he cares deeply about the subject matter. So will you after seeing this powerful, shocking documentary.

Fin is streaming exclusively on discovery+.

out of four

Fin is unrated, but contains adult language and extremely graphic footage of sharks being killed/maimed. The running time is 1 hour and 36 minutes.