Volcanoes turn out to be a solar-system-wide phenomenon, not just an Earth feature, and this Horizon episode tracks that idea from Iceland's lava fields out to the moons and planets where eruptions dwarf anything terrestrial. Geologists working the Icelandic landscape explain how lava flows, gas plumes, and crater formation here provide a working model for reading volcanic activity elsewhere in the Solar System. The film moves through the extremes: Jupiter's moon Io, the most volcanically active body known, sulfur volcanoes erupting on Neptune's moon Triton, and the ancient, colossal shield volcano Olympus Mons on Mars, roughly three times the height of Everest. Scientists use these comparisons to argue that studying alien vulcanism sharpens what we know about Earth's own geological history and even the conditions that could have let life begin here. Footage mixes ground-level Icelandic fieldwork with spacecraft imagery and animation of planetary surfaces, narrated in the plain, question-driven style typical of the Horizon series.