This episode picks up the story at another moment of buoyant optimism in the cancer world: Scientists believe they have cracked the essential mystery of the malignant cell and the first targeted therapies have been developed, with the promise of many more to follow. But very quickly cancer reveals new layers of complexity and a formidable array of unforeseen defenses. In the disappointment that follows, many call for a new focus on prevention and early detection as the most promising fronts in the war on cancer.