Ocean acidification gets treated here as a slow-motion chemistry experiment running on a planetary scale: as seawater absorbs more carbon dioxide, its pH drops, and marine life built on calcium carbonate shells and skeletons starts to struggle. Explorer Paul Rose leads a team of specialists through expeditions built around that question, with maritime archaeologist Lucy Blue examining shipwrecks and sunken settlements for clues to how humans and the sea have interacted over centuries, marine biologist Tooni Mahto tracking present-day ocean life, and environmentalist Philippe Cousteau Jr looking at how the water is changing now and what that means going forward. The team's stops include waters beneath Arctic ice sheets and the submerged sinkholes off the Caribbean. Sigourney Weaver narrates, framing acidification as a shift with no real precedent in tens of millions of years. Produced for the NRDC and originally broadcast on Discovery Planet Green, the film argues that the oceans, which regulate climate and support most of the planet's biodiversity, remain less studied than the surface of Mars.