Every two years Mars swings within about 36 million miles of Earth, close enough for astronomers to take their best look at the planet, and this film traces what that scrutiny has actually found. It starts with Giovanni Schiaparelli's 1877 sighting of channel-like markings, which Percival Lowell turned into a public sensation, arguing in three bestselling books that intelligent Martians had built canals to move meltwater across a dying world. The 1964 Mariner 4 flyby ended that fantasy, returning images of a cratered, moon-like surface, and later Mariner missions found dormant volcanoes and Valles Marineris, the deepest, longest canyon in the solar system. The twin Viking landers of the mid-1970s tested Martian soil directly for life and instead confirmed a planet hostile to it, with air temperatures ranging from -20 to -200 degrees Fahrenheit and less moisture in its atmosphere than a fingernail's thickness of frost. Yet the same orbiters photographed ancient riverbeds and shorelines, leaving the film's real question open: where did all that water go?