In 2006, Russian FSB (formerly KGB) defector Alexander Litvinenko was the victim of a rare and frightening poison: a tiny speck of polonium-210, delivered in a cup of tea, was enough to destroy his body from the inside out. On Medium, Will Storr follows the story of Litvinenko's death, from his work for and subsequent disillusionment with Russia's secret service, to the mechanism that makes polonium-210 such an effective poison, to the trail of radioactivity that allowed British investigators to track the poison to a potential suspect — although Litvinenko's death still remains unsolved today, partly because of the many enemies he made in his departure from Russia and the FSB.