Chris Lintott, Professor of Astrophysics at Oxford and presenter of the BBC's Sky at Night, traces the history behind the 2006 International Astronomical Union vote that reclassified Pluto as a dwarf planet. He starts with the mathematical prediction and discovery of Neptune, Percival Lowell's search for a hypothetical Planet X, and Clyde Tombaugh's 1930 discovery of Pluto itself. The lecture follows the growing doubts raised by Pluto's unusual orbit, the identification of its moon Charon, and the subsequent discovery of the Kuiper Belt and the larger object Eris, which forced astronomers to settle on a formal definition of a planet. Lintott covers the chaotic 2006 IAU vote and closes with what the New Horizons flyby revealed up close. Along the way he asks what it means to name a class of object in science, and argues Pluto remains worth studying regardless of its label.