A documentary by Rob Stewart. Rated G.

Rob Stewart’s 2006 shark-finning documentary, Sharkwater, had the focus of a hammerhead on the hunt. His follow-up, Revolution, is an unwieldy mix of Cousteau-quality underwater photography, jaunts to tropical rain forests and tar sands, expert interviews, and trips to climate-change protests around the globe. The glue that holds it all together is affable surfer-dude Stewart.

Revolution starts undersea, showing the delicate ecosystem in all its ethereal splendour: sailfish skewering a shimmering bait ball or coral coming alive to eat zooplankton like glowing, monster-movie blobs. But it’s above the ocean, during a promotional tour for Sharkwater, that Stewart’s focus changes: a woman at a talkback in Hong Kong asks him what the point is in trying to save sharks when the UN predicts fisheries are all going to be wiped out by 2048. “This is no longer about saving the oceans,” Stewart says. “It’s about saving ourselves.”

Cue Stewart’s embracing of the climate-change movement, with trips to Alberta’s postapocalyptic tar sands, Madagascar’s starving aid-dependent towns, and countless protests. This is highly subjective filmmaking, but, driven by a haunting, atmospheric score, it moves quickly, putting the usually mind-numbing science of climate change in digestible form without sounding preachy.

Revolution’s big problem is that it ends up so far away from where it started; by the end, it has switched from spectacular undersea voyage to earnest, there-is-hope-in-“the youth” activism. Stewart is so damn persuasive that you want to believe him when he starts showing the critical mass rising against global warming. But in the last minutes of the documentary, around the time poster slogans like “Get educated” and “Listen to youth” start flashing on-screen, the more jaded will think back to the utter frustration of scientists and veteran activists who appear earlier in the film. As one puts it: “What does it take? If it was reason, we would have done it by now.”