The story starts in Flagstaff, Arizona, where astronomer Percival Lowell built an observatory in the early 1900s and became convinced that canals crossed the surface of Mars, evidence, he thought, of a civilization moving water across a drying planet. That obsession opens a wider question the film pursues: how did any planet, including Earth, end up with oceans at all. Planetary scientists walk through the leading theories, comets delivering ice from the outer solar system, water trapped in the rocky material that formed Earth itself, and asteroids crashing in during the early bombardment period. Footage of telescopes, spacecraft data, and lab experiments on meteorite samples back up the competing claims, while the film compares the chemical signatures scientists use to trace water's origin, particularly the ratio of hydrogen isotopes in comets versus what is found in Earth's oceans. Lowell's Mars theory turned out to be wrong, but the film treats his question as the right one, using it to frame a debate that current missions to comets and asteroids are still trying to settle.